Optical modulator, optical waveguide device and acousto-optic tunable filter apparatus

ABSTRACT

The invention provides an optical modulator wherein appearance of radiation mode light can be suppressed to improve performances as a device. The optical modulator includes a multiplexing waveguide which in turn includes two input waveguides to which propagation lights from a linear waveguide are inputted, a waveguide coupling element, and two output waveguides. The width of the waveguide coupling element on the input side is greater than the total width of the two input waveguides, and the width of the waveguide coupling element on the output side is greater than the total width of the two output waveguides. The device of the present invention is applied typically to an optical communication system.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to an optical modulator and an optical waveguide device suitable for use with an optical communication system.

[0003] 2. Description of the Related Art

[0004]FIG. 16 is a schematic view showing a conventional optical multiplexer/demultiplexer having two inputs and two outputs. The optical multiplexer/demultiplexer 700 shown in FIG. 16 includes two input waveguides 702 and 703, a waveguide coupling element 706 and two output waveguides 704 and 705 formed on a substrate 701. The optical multiplexer/demultiplexer 700 can multiplex or demultiplex, when input light propagating in the input waveguides 702 and/or 703 is to be outputted to the output waveguides 704 or/and 705 through the waveguide coupling element 706, the input light.

[0005] In the optical multiplexer/demultiplexer 700, for example, input light P₀ inputted to the input waveguide 702 is demultiplexed into and outputted as output lights P₁ and P₂ from the output waveguides 704 and 705 through the waveguide coupling element 706.

[0006] Here, in FIG. 16, the input and output waveguides 702 to 705 have an equal width w₀, and they are connected with a width d left therebetween to the waveguide coupling element 706. Further, the two input waveguides 702 and 703 and the two output waveguides 704 and 705 are formed individually with an angle θ defined therebetween such that the distance therebetween decreases toward the waveguide coupling element 706.

[0007] The connection angle θ of the input and output waveguides 702 to 705 to the interference waveguide 706 is sufficiently small. Therefore, since the widths A and B of the connection points between the input and output waveguides 702 to 705 and the waveguide coupling element 706 can be approximated as an equal value to the input and output waveguide width w₀, the width w_(w) of the waveguide coupling element 706 can be represented as 2w₀+d. It is to be noted that, in FIG. 16, reference character Lc denotes the length of the waveguide coupling element 706 in its longitudinal direction (propagation direction of light).

[0008] For example, if the length Lc is adjusted to form the waveguide coupling element 706 as shown in FIG. 17, then the characteristic as the optical multiplexer/demultiplexer 700, that is, the multiplexing/demultiplexing characteristic for input light, can be varied.

[0009] In particular, as shown in FIG. 17, where the length Lc is equal to L₁, the ratio [P₁/{P₁+P₂}] of the output light P₁ to the overall output light {P₁+P₂} is equal to 1. Therefore, the output light from the optical multiplexer/demultiplexer 700 is outputted as the output light P₁ only from the output waveguide 704.

[0010] On the other hand, where the length Lc is equal to L2, since the ratio P₁/{P₁+P₂} is equal to 0.5, the branching ratio of the input light P₀ at the output waveguides 704 and 705 is 1:1, and the output light from the optical multiplexer/demultiplexer 700 is outputted as the output lights P₁ and P₂ from the output waveguides 704 and 705, respectively. In other words, the optical multiplexer/demultiplexer 700 can be used as a 3 dB coupler.

[0011] Further, where the length Lc is equal to L3, since the ratio P₁/{P₁+P₂} is equal to 0, the output light from the optical multiplexer/demultiplexer 700 is outputted as the output light P₂ only from the output waveguide 705.

[0012] The optical multiplexer/demultiplexer 700 having such a configuration as described above can be used as 3 dB couplers 700A and 700B, for example, in such a Mach-Zehnder type modulator 710 as shown in FIG. 18. Here, the Mach-Zehnder type optical modulator 710 shown in FIG. 18 includes a Mach-Zehnder type optical waveguide 714 formed from 3 dB couplers 700A and 700B and two linear waveguides 712 and 713 on a substrate 711, and further includes a signal electrode 715 and a ground electrode 716 formed on the substrate 711.

[0013] In the Mach-Zehnder type optical modulator 710 having the configuration described above, input light inputted through an input waveguide 702A from between two input waveguides 702A and 703A which form the 3 dB coupler 700A is branched at a ratio of 1:1, and the branched lights propagate in the linear optical waveguides 712 and 713. It is to be noted that, by a variation of an electric field by a signal voltage applied to the signal electrode 715, light modulated in accordance with the signal voltage just mentioned can be outputted from the output waveguides 704B and 705B.

[0014] Further, the lights propagating in the linear waveguides 712 and 713 are inputted to the input waveguides 702B and 703B which form the 3 dB coupler 700B, respectively, and then multiplexed by the waveguide coupling element 706B, whereafter the multiplexed light is outputted from both of the output waveguides 704B and 705B. Particularly, the output light from the output waveguide 704B is used as monitor light, and the output light from the output waveguide 705B is used as signal light.

[0015] However, in such a conventional optical waveguide device as described above, a great difference exists between a propagation mode in the input and output waveguides and a propagation mode in the waveguide coupling element, and therefore, there is a subject that an radiation mode appear.

[0016] Further, also where an optical waveguide device from which such radiation mode light as just described is generated is applied to a device such as an optical modulator, a wavelength filter or the like, there is the possibility that, by generation of the radiation mode described above, the performance improvement of the device itself may be hindered.

[0017] Particularly, where the optical waveguide device 700 having such a configuration as described above is applied as the 3 dB couplers 700A and 700B to such a Mach-Zehnder type optical modulator 700 as described above with reference to FIG. 18, radiation mode light is generated in the 3 dB coupler 700B described above. Therefore, a deviation δ appears between a voltage value V₁ when the signal light (refer to a full line waveform in FIG. 19) is in an on state and a voltage value V₂ at an off point of the monitor light (refer to a broken line waveform in FIG. 19) as shown in FIG. 19 although they are desirably to be coincident with each other. The ratio [(V₂−V₁)/V_(π)] of the deviation to the half-wavelength voltage V_(π) is approximately 10%, and this makes a hindrance to use of the output light of the conventional optical waveguide device 700 as monitor light for adjustment of the operating point.

SUMMARY OF THE INVENTION

[0018] It is an object of the present invention to provide an optical modulator and an optical waveguide device wherein production of radiation mode light can be suppressed to improve performances as a device.

[0019] In order to attain the object described above, according to an aspect of the present invention, there is provided an optical waveguide device, comprising two input waveguides, a waveguide coupling element for coupling the two input waveguides with a first distance, and two output waveguides branched from the waveguide coupling element with a second distance, the first distance in the waveguide coupling element being set to one half or less of width at a portion at which each of the input waveguides is connected to the waveguide coupling element, width of the waveguide coupling element on the input waveguides side being set to 2.5 times to 3.5 times the width at the portion at which each of the input waveguides is connected to the waveguide coupling element.

[0020] According to another aspect of the present invention, there is provided an optical waveguide device, comprising two input waveguides, a waveguide coupling element for coupling the two input waveguides with a first distance, and two output waveguides branched from the waveguide coupling element with a second distance, the second distance in the waveguide coupling element being set to one half or less of width at a portion at which each of the output waveguides is connected to the waveguide coupling element, width of the waveguide coupling element on the output waveguides side being set to 2.5 times to 3.5 times the width at the portion at which each of the output waveguides is connected to the waveguide coupling element.

[0021] According to a further aspect of the present invention, there is provided an optical waveguide device, comprising a substrate having an electro-optical effect, a polarization wave separating waveguide formed on the substrate for separating propagation light into polarization waves of two polarization modes of TM and TE, two linear waveguides formed on the substrate for individually propagating the propagation lights separated by the polarization wave separating waveguide, and a polarization wave synthesis waveguide formed on the substrate for synthesizing the propagation lights from the two linear waveguides, the polarization wave separating waveguide including two input waveguides, a waveguide coupling element, and two output waveguides, width of the waveguide coupling element on the input side being set greater than the width of the two input waveguides, width of the waveguide coupling element on the output side being set greater than the width of the two output waveguides.

[0022] According a still further aspect of the present invention, there is provided an optical modulator, comprising a substrate, and an optical device provided on the substrate and including a branching waveguide for branching propagation light into two lights, two linear waveguides for propagating the propagation lights branched by the branching waveguide, and a synthesis waveguide for synthesizing the propagation lights from the two linear waveguides such that a phase difference is provided to the propagation lights in the two linear waveguides, the synthesis waveguide including two input waveguides for receiving the propagation lights from the two linear waveguides as inputs thereto, a waveguide coupling element, and two output waveguides, width of the waveguide coupling element on the input side being set greater than the width of the two input waveguides, width of the waveguide coupling element on the output side being set greater than the width of the two output waveguides.

[0023] According to a yet further aspect of the present invention, there is provided an optical waveguide device, comprising two input waveguides, a waveguide coupling element for coupling the two input waveguides with a distance d₁, and two output waveguides branched from the waveguide coupling element with a distance d₂, the distance d₁ in the waveguide coupling element being set to one half or less of width w₀₁ of a portion at which each of the input waveguides is connected to the waveguide coupling element, width w_(w1) of the waveguide coupling element on the input waveguides side being set to 2.5 times to 3.5 times the width w₀₁.

[0024] The optical waveguide device may be configured such that the distance d₂ in the waveguide coupling element is set to one half or less of width w₀₂ of a portion at which each of the output waveguides is connected to the waveguide coupling element, and width w_(w2) of the waveguide coupling element on the output waveguides side is set to 2.5 times to 3.5 times the width w₀₂.

[0025] According to a yet further aspect of the present invention, there is provided an optical waveguide device, comprising two input waveguides, a waveguide coupling element for coupling the two input waveguides with a distance d₁, and two output waveguides branched from the waveguide coupling element with a distance d₂, the distance d₂ in the waveguide coupling element being set to one half or less of width w₀₂ of a portion at which each of the output waveguides is connected to the waveguide coupling element, width w_(w2) of the waveguide coupling element on the output waveguides side being set to 2.5 times to 3.5 times the width w₀₂.

[0026] According to a yet further aspect of the present invention, there is provided an optical waveguide device, comprising two input waveguides, a waveguide coupling element for coupling the two input waveguides with a distance d₁, and two output waveguides branched from the waveguide coupling element with a distance d₂, width w_(w1) of the waveguide coupling element on the input waveguides side being greater than the sum 2w₀₁+d₁ of the sum 2w₀₁ of widths of portions at which the two input waveguides are connected to the waveguide coupling element and the distance d₁ so that radiation mode light can be suppressed better than where the width w_(w1) is equal to the sum 2w₀₁+d₁.

[0027] The optical waveguide device may be configured such that width w_(w2) of the waveguide coupling element on the output waveguides side is greater than the sum 2w₀₂+d₂ of the sum 2w₀₂ of widths of portions at which the two output waveguides are connected to the waveguide coupling element and the distance d₂ so that radiation mode light can be suppressed better than where the width w_(w2) is equal to the sum 2w₀₂+d₂.

[0028] According to a yet further aspect of the present invention, there is provided an optical waveguide device, comprising two input waveguides, a waveguide coupling element for coupling the two input waveguides with a distance d₁, and two output waveguides branched from the waveguide coupling element with a distance d₂, width w_(w2) of the waveguide coupling element on the output waveguides side being greater than the sum 2w₀₂+d₂ of the sum 2w₀₂ of widths of portions at which the two output waveguides are connected to the waveguide coupling element and the distance d₂ so that radiation mode light can be suppressed better than where the width w_(w2) is equal to the sum 2w₀₂+d₂.

[0029] According to a yet further aspect of the present invention, there is provided an optical waveguide device, comprising a substrate having an electro-optical effect, and a Mach-Zehnder type optical waveguide formed on the substrate and including a branching waveguide for branching propagation light into two propagation lights, two linear waveguides for propagating the two propagation lights branched by the branching waveguide, and a multiplexing waveguide for multiplexing the propagation lights from the two linear waveguides, the multiplexing waveguide including two input waveguides for inputting the propagation lights from the linear waveguides, a waveguide coupling element for coupling the two input waveguides with a distance d₁, and two output waveguides branched from the waveguide coupling element with another distance d₂, the distance d₁ in the waveguide coupling element being set to one half or less of width w₀₁ at a portion at which each of the input waveguides is connected to the waveguide coupling element, width w_(w1) of the waveguide coupling element on the input waveguides side being set to 2.5 times to 3.5 times the width w₀₁, the length of the waveguide coupling element being set so that light propagating in one of the two input waveguides is branched to the two output waveguides with a power radio of 1:1.

[0030] According to a yet further aspect of the present invention, there is provided an optical waveguide device, comprising a substrate having an electro-optical effect, and a Mach-Zehnder type optical waveguide formed on the substrate and including a branching waveguide for branching propagation light into two propagation lights, two linear waveguides for propagating the two propagation lights branched by the branching waveguide, and a multiplexing waveguide for multiplexing the propagation lights from the two linear waveguides, the multiplexing waveguide including two input waveguides for inputting the propagation lights from the linear waveguides, a waveguide coupling element for coupling the two input waveguides with distance d₁, and two output waveguides branched from the waveguide coupling element with another distance d₂, the distance d₂ in the waveguide coupling element being set to one half or less of width w₀₂ at a portion at which each of the output waveguides is connected to the waveguide coupling element, width w_(w2) of the waveguide coupling element on the output waveguides side being set to 2.5 times to 3.5 times the width w₀₂, the length of the waveguide coupling element being set so that light propagating in one of the two input waveguides is branched to the two output waveguides with a power radio of 1:1.

[0031] According to a yet further aspect of the present invention, there is provided an optical waveguide device, comprising a substrate having an electro-optical effect, a polarization wave separating waveguide formed on the substrate for separating propagation light into polarization waves of polarization modes of TM and TE, two linear waveguides formed on the substrate for individually propagating the propagation lights separated by the polarization wave separating waveguide, and a polarization wave synthesis waveguide formed on the substrate for synthesizing the propagation lights from the two linear waveguides, the polarization wave separating waveguide including two input waveguides, a waveguide coupling element for coupling the two input waveguides with a distance d₁, and two output waveguides branched from the waveguide coupling element with another distance d₂, the distance d₁ in the waveguide coupling element being set to one half or less of width w₀₁ at a portion at which each of the input waveguides is connected to the waveguide coupling element, width w_(w1) of the waveguide coupling element on the input waveguides side being set to 2.5 times to 3.5 times the width w₀₁, the length of the waveguide coupling element being set so that, where polarization waves of the two polarization modes of TM and TE are inputted from one of the two input waveguides, the polarization waves may individually be outputted to different ones of the output waveguides from each other.

[0032] According to a yet further aspect of the present invention, there is provided an optical waveguide device, comprising a substrate having an electro-optical effect, a polarization wave separating waveguide formed on the substrate for separating propagation light into polarization waves of polarization modes of TM and TE, two linear waveguides formed on the substrate for individually propagating the propagation lights separated by the polarization wave separating waveguide, and a polarization wave synthesis waveguide formed on the substrate for synthesizing the propagation lights from the two linear waveguides, the polarization wave separating waveguide including two input waveguides, a waveguide coupling element for coupling the two input waveguides with a distance d₁, and two output waveguides branched from the waveguide coupling element with another distance d₂, the distance d₂ in the waveguide coupling element being set to one half or less of width w₀₂ at a portion at which each of the output waveguides is connected to the waveguide coupling element, width w_(w2) of the waveguide coupling element on the output waveguides side being set to 2.5 times to 3.5 times the width w₀₂, the length of the waveguide coupling element being set so that, where polarization waves of the two polarization modes of TM and TE are inputted from one of the two input waveguides, the polarization waves may individually be outputted to different ones of the output waveguides from each other.

[0033] According to a yet further aspect of the present invention, there is provided an optical waveguide device, comprising a substrate having an electro-optical effect, a polarization wave separating waveguide formed on the substrate for separating propagation light into polarization waves of polarization modes of TM and TE, two linear waveguides formed on the substrate for individually propagating the propagation lights separated by the polarization wave separating waveguide, and a polarization wave synthesis waveguide formed on the substrate for synthesizing the propagation lights from the two linear waveguides, the polarization wave synthesis waveguide including two input waveguides for receiving the propagation lights from the two linear waveguides as inputs thereto, a waveguide coupling element for coupling the two input waveguides with a distance d₁, and two output waveguides branched from the waveguide coupling element with another distance d₂, the distance d₁ in the waveguide coupling element being set to one half or less of width w₀₁ at a portion at which each of the input waveguides is connected to the waveguide coupling element, width w_(w1) of the waveguide coupling element on the input waveguides side being set to 2.5 times to 3.5 times the width w₀₁, the length of the waveguide coupling element being set so that, where a polarization wave of the polarization mode of TM is inputted from one of the two input waveguides and another polarization wave of the polarization mode of TE is inputted from the other of the two input waveguides, the polarization waves may be synthesized by the waveguide coupling element and the synthesized polarization light may be outputted to one of the output waveguides.

[0034] According to a yet further aspect of the present invention, there is provided an optical waveguide device, comprising a substrate having an electro-optical effect, a polarization wave separating waveguide formed on the substrate for separating propagation light into polarization waves of polarization modes of TM and TE, two linear waveguides formed on the substrate for individually propagating the propagation lights separated by the polarization wave separating waveguide, and a polarization wave synthesis waveguide formed on the substrate for synthesizing the propagation lights from the two linear waveguides, the polarization wave synthesis waveguide including two input waveguides for receiving the propagation lights from the two linear waveguides as inputs thereto, a waveguide coupling element for coupling the two input waveguides with a distance d₁, and two output waveguides branched from the waveguide coupling element with another distance d₂, the distance d₂ in the waveguide coupling element being set to one half or less of width w₀₂ at a portion at which each of the output waveguides is connected to the waveguide coupling element, width w_(w2) of the waveguide coupling element on the output waveguides side being set to 2.5 times to 3.5 times the width w₀₂, the length of the waveguide coupling element being set so that, where a polarization wave of the polarization mode of TM is inputted from one of the two input waveguides and another polarization wave of the polarization mode of TE is inputted from the other of the two input waveguides, the polarization waves may be synthesized by the waveguide coupling element and the synthesized polarization wave may be outputted from one of the output waveguides.

[0035] According to a yet further aspect of the present invention, there is provided an optical modulator, comprising a substrate having an electro-optical effect, a Mach-Zehnder type optical waveguide formed on the substrate, a signal electrode section provided on the optical waveguide for being supplied with a signal voltage, a bias electrode section provided on the optical waveguide for being supplied with a bias voltage, a coupler provided on the outgoing side of the Mach-Zehnder type optical waveguide for fetching monitor light, a monitor photodiode for detecting a light signal of the monitor light fetched by the coupler, and a bias voltage control section for controlling the bias voltage to be supplied to the bias electrode section based on the light signal detected by the monitor photodiode, the coupler including two input waveguides through one of which light from the Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling the two input waveguides with a distance d₁, and a signal light propagating output waveguide and a monitoring output waveguide branched from the waveguide coupling element with a distance d₂, the distance d₁ in the waveguide coupling element being set to one half or less of width w₀₁ at a portion at which each of the input waveguides is connected to the waveguide coupling element, width w_(w1) of the waveguide coupling element on the input waveguides side being set to 2.5 times to 3.5 times the width w₀₁, the length of the waveguide coupling element being set so that light which propagates in one of the two input waveguides may be branched to the two output waveguides with a power radio of 1:1.

[0036] According to a yet further aspect of the present invention, there is provided an optical modulator, comprising a substrate having an electro-optical effect, a Mach-Zehnder type optical waveguide formed on the substrate, a signal electrode section provided on the optical waveguide for being supplied with a signal voltage, a bias electrode section provided on the optical waveguide for being supplied with a bias voltage, a coupler provided on the outgoing side of the Mach-Zehnder type optical waveguide for fetching monitor light, a monitor photodiode for detecting a light signal of the monitor light fetched by the coupler, and a bias voltage control section for controlling the bias voltage to be supplied to the bias electrode section based on the light signal detected by the monitor photodiode, the coupler including two input waveguides through one of which light from the Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling the two input waveguides with a distance d₁, and a signal light propagating output waveguide and a monitoring output waveguide branched from the waveguide coupling element with a distance d₂, the distance d₂ in the waveguide coupling element being set to one half or less of width w₀₂ at a portion at which each of the output waveguides are connected to the waveguide coupling element, width w_(w2) of the waveguide coupling element on the input waveguides side being set to 2.5 times to 3.5 times the width w₀₂, the length of the waveguide coupling element being set so that light which propagates one of the two input waveguides may be branched to the two output waveguides with a power radio of 1:1.

[0037] According to a yet further aspect of the present invention, there is provided an optical modulator, comprising a substrate having an electro-optical effect, first and second Mach-Zehnder type optical waveguides formed in series on the substrate, a first signal electrode section provided on the first Mach-Zehnder type optical waveguide for being supplied with a clock signal, a second signal electrode section provided on the second Mach-Zehnder type optical waveguide for being supplied with a data signal, a first bias electrode section provided on the first Mach-Zehnder type optical waveguide for being supplied with a bias voltage, a second bias electrode section provided on the second Mach-Zehnder type optical waveguide for being supplied with another bias voltage, a first coupler provided on the outgoing side of the first Mach-Zehnder type optical waveguide for fetching monitor light, a second coupler provided on the outgoing side of the second Mach-Zehnder type optical waveguide for fetching monitor light, a first monitor photodiode for detecting a light signal of the monitor light fetched by the first coupler, a second monitor photodiode for detecting a light signal of the monitor light fetched by the second coupler, a first bias voltage control section for controlling the bias voltage to be supplied to the first bias electrode section based on the light signal detected by the first monitor photodiode, and a second bias voltage control section for controlling the bias voltage to be supplied to the second bias electrode section based on the light signal detected by the second monitor photodiode, the first coupler including two input waveguides through one of which light from the first Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling the two input waveguides with a distance d₁₁, and two output waveguides branched from the waveguide coupling element with a distance d₂₁, the distance d₁₁ in the waveguide coupling element of the first coupler being set to one half or less of width w₀₁₁ at a portion at which each of the input waveguides is connected to the waveguide coupling element, width w_(w11) of the waveguide coupling element on the input waveguides side being set to 2.5 times to 3.5 times the width w₀₁₁.

[0038] According to a yet further aspect of the present invention, there is provided an optical modulator, comprising a substrate having an electro-optical effect, first and second Mach-Zehnder type optical waveguides formed in series on the substrate, a first signal electrode section provided on the first Mach-Zehnder type optical waveguide for being supplied with a clock signal, a second signal electrode section provided on the second Mach-Zehnder type optical waveguide for being supplied with a data signal, a first bias electrode section provided on the first Mach-Zehnder type optical waveguide for being supplied with a bias voltage, a second bias electrode section provided on the second Mach-Zehnder type optical waveguide for being supplied with another bias voltage, a first coupler provided on the outgoing side of the first Mach-Zehnder type optical waveguide for fetching monitor light, a second coupler provided on the outgoing side of the second Mach-Zehnder type optical waveguide for fetching monitor light, a first monitor photodiode for detecting a light signal of the monitor light fetched by the first coupler, a second monitor photodiode for detecting a light signal of the monitor light fetched by the second coupler, a first bias voltage control section for controlling the bias voltage to be supplied to the first bias electrode section based on the light signal detected by the first monitor photodiode, and a second bias voltage control section for controlling the bias voltage to be supplied to the second bias electrode section based on the light signal detected by the second monitor photodiode, the first coupler including two input waveguides through one of which light from the first Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling the two input waveguides with a distance d₁₁, and two output waveguides branched from the waveguide coupling element with a distance d₂₁, the distance d₂₁ in the waveguide coupling element of the first coupler being set to one half or less of width w₀₂₁ at a portion at which each of the output waveguides is connected to the waveguide coupling element, width w_(w21) of the waveguide coupling element on the output waveguides side being set to 2.5 times to 3.5 times the width w₀₂₁.

[0039] According to a yet further aspect of the present invention, there is provided an optical modulator, comprising a substrate having an electro-optical effect, first and second Mach-Zehnder type optical waveguides formed in series on the substrate, a first signal electrode section provided on the first Mach-Zehnder type optical waveguide for being supplied with a clock signal, a second signal electrode section provided on the second Mach-Zehnder type optical waveguide for being supplied with a data signal, a first bias electrode section provided on the first Mach-Zehnder type optical waveguide for being supplied with a bias voltage, a second bias electrode section provided on the second Mach-Zehnder type optical waveguide for being supplied with another bias voltage, a first coupler provided on the outgoing side of the first Mach-Zehnder type optical waveguide for fetching monitor light, a second coupler provided on the outgoing side of the second Mach-Zehnder type optical waveguide for fetching monitor light, a first monitor photodiode for detecting a light signal of the monitor light fetched by the first coupler, a second monitor photodiode for detecting a light signal of the monitor light fetched by the second coupler, a first bias voltage control section for controlling the bias voltage to be supplied to the first bias electrode section based on the light signal detected by the first monitor photodiode, and a second bias voltage control section for controlling the bias voltage to be supplied to the second bias electrode section based on the light signal detected by the second monitor photodiode, the second coupler including two input waveguides through one of which light from the second Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling the two input waveguides with a distance d₁₂, and two output waveguides branched from the waveguide coupling element with a distance d₂₂, the distance d₁₂ in the waveguide coupling element of the second coupler being set to one half or less of width w₀₁₂ at a portion at which the input waveguides are connected to the waveguide coupling element, width w_(w12) of the waveguide coupling element on the input waveguides side being set to 2.5 times to 3.5 times the width w₀₁₂.

[0040] According to a yet further aspect of the present invention, there is provided an optical modulator, comprising a substrate having an electro-optical effect, first and second Mach-Zehnder type optical waveguides formed in series on the substrate, a first signal electrode section provided on the first Mach-Zehnder type optical waveguide for being supplied with a clock signal, a second signal electrode section provided on the second Mach-Zehnder type optical waveguide for being supplied with a data signal, a first bias electrode section provided on the first Mach-Zehnder type optical waveguide for being supplied with a bias voltage, a second bias electrode section provided on the second Mach-Zehnder type optical waveguide for being supplied with another bias voltage, a first coupler provided on the outgoing side of the first Mach-Zehnder type optical waveguide for fetching monitor light, a second coupler provided on the outgoing side of the second Mach-Zehnder type optical waveguide for fetching monitor light, a first monitor photodiode for detecting a light signal of the monitor light fetched by the first coupler, a second monitor photodiode for detecting a light signal of the monitor light fetched by the second coupler, a first bias voltage control section for controlling the bias voltage to be supplied to the first bias electrode section based on the light signal detected by the first monitor photodiode, and a second bias voltage control section for controlling the bias voltage to be supplied to the second bias electrode section based on the light signal detected by the second monitor photodiode, the second coupler including two input waveguides through one of which light from the second Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling the two input waveguides with a distance d₁₂, and two output waveguides branched from the waveguide coupling element with a distance d₂₂, the distance d₂₂ in the waveguide coupling element of the second coupler being set to one half or less of width w₀₂₂ at a portion at which each of the output waveguides is connected to the waveguide coupling element, width w_(w22) of the waveguide coupling element on the output waveguides side being set to 2.5 times to 3.5 times the width w₀₂₂.

[0041] According to a yet further aspect of the present invention, there is provided an optical modulator, comprising a substrate having an electro-optical effect, first and second Mach-Zehnder type optical waveguides formed in series on the substrate, a first signal electrode section provided on the first Mach-Zehnder type optical waveguide for being supplied with a clock signal, a second signal electrode section provided on the second Mach-Zehnder type optical waveguide for being supplied with a data signal, a first bias electrode section provided on the first Mach-Zehnder type optical waveguide for being supplied with a bias voltage, a second bias electrode section provided on the second Mach-Zehnder type optical waveguide for being supplied with another bias voltage, a first coupler provided on the outgoing side of the first Mach-Zehnder type optical waveguide for fetching monitor light, a second coupler provided on the outgoing side of the second Mach-Zehnder type optical waveguide for fetching monitor light, a first monitor photodiode for detecting a light signal of the monitor light fetched by the first coupler, a second monitor photodiode for detecting a light signal of the monitor light fetched by the second coupler, a first bias voltage control section for controlling the bias voltage to be supplied to the first bias electrode section based on the light signal detected by the first monitor photodiode, and a second bias voltage control section for controlling the bias voltage to be supplied to the second bias electrode section based on the light signal detected by the second monitor photodiode, the first coupler including two first input waveguides through one of which light from the first Mach-Zehnder type optical waveguide is inputted, a first waveguide coupling element for coupling the two first input waveguides with a distance d₁₁, and two first output waveguides branched from the first waveguide coupling element with a distance d₂₁, the second coupler including two second input waveguides through one of which light from the second Mach-Zehnder type optical waveguide is inputted, a second waveguide coupling element for coupling the two second input waveguides with a distance d₁₂, and two second output waveguides branched from the second waveguide coupling element with a distance d₂₂, the distances d₁₁ and d₂₁ in the first waveguide coupling element of the first coupler being set to one half or less of widths w₀₁₁ and w₀₂₁ at portions at which each of the first input waveguides and each of the first output waveguides are connected to the first waveguide coupling element, respectively, the widths w_(w11) and w_(w21) of the first waveguide coupling element on the first input waveguides side and the first output waveguides side being set to 2.5 times to 3.5 times the widths w₀₁₁ and w₀₂₁, respectively, the distances d₁₂ and d₂₂ in the second waveguide coupling element of the second coupler being set to one half or less of widths w₀₁₂ and w₀₂₂ at portions at which each of the second input waveguides and each of the second output waveguides are connected to the second waveguide coupling element, respectively, the widths w_(w12) and w_(w22) of the second waveguide coupling element on the second input waveguides side and the second output waveguides side being set to 2.5 times to 3.5 times the widths w₀₁₂ and w₀₂₂, respectively.

[0042] According to a yet further aspect of the present invention, there is provided an acousto-optic tunable filter apparatus, comprising a substrate, and first and second acousto-optic tunable filters formed in series on the substrate and utilizing an acousto-optic effect, each of the first and second acousto-optic tunable filters including an optical waveguide which in turn includes a first polarization beam splitter for separating propagation light into polarization waves of polarization modes of TM and TE, two linear waveguides for individually propagating the propagation lights separated by the first polarization beam splitter, and a second polarization beam splitter for synthesizing the propagation lights from the two linear waveguides, each of the first and second acousto-optic tunable filters further including an electrode for receiving an RF signal to be used to generate a surface acoustic wave for the acousto-optic effect on the optical waveguide, the first polarization beam splitter including two polarization wave separating input waveguides, a polarization wave separating waveguide coupling element for coupling the two polarization wave separating input waveguides with a distance d₁₃ so that lights propagating in the two polarization wave separating input waveguides interfere with each other, and two polarization wave separating output waveguides branched from the polarization wave separating waveguide coupling element with a distance d₂₃, the second polarization beam splitter including two polarization wave synthesizing input waveguides, a polarization wave synthesizing waveguide coupling element for coupling the two polarization wave synthesizing input waveguides with a distance d₁₄ so that lights propagating in the two polarization wave synthesizing input waveguides interfere with each other, and two polarization wave synthesizing output waveguides branched from the polarization wave synthesizing waveguide coupling element with a distance d₂₄, the distances d₁₃ and d₂₃ being set to one half or less of widths w₀₁₃ and w₀₂₃ at portions at which each of the polarization wave separating input waveguides and each of the polarization wave separating output waveguides are connected to the polarization wave separating waveguide coupling element, respectively, widths w_(w13) and w_(w23) of the polarization wave separating waveguide coupling element on the polarization wave separating input waveguides and the polarization wave separating output waveguides sides being set to 2.5 times to 3.5 times the widths w₀₁₃ and w₀₂₃, respectively, the distances d₁₄ and d₂₄ being set to one half or less of widths w₀₁₄ and w₀₂₄ at portions at which each of the polarization wave synthesizing input waveguides and each of the polarization wave synthesizing output waveguides are connected to the polarization wave synthesizing waveguide coupling element, respectively, widths w_(w14) and w_(w24) of the polarization wave synthesizing waveguide coupling element on the polarization wave synthesizing input waveguides and the polarization wave synthesizing output waveguides sides being set to 2.5 times to 3.5 times the widths w₀₁₄ and w₀₂₄, respectively.

[0043] With the optical modulators, optical waveguide devices and acousto-optical filter apparatus according to the present invention, the following advantages can be achieved.

[0044] 1. Since the multiplexing waveguide is configured such that it includes two input waveguides for receiving propagation lights from the linear waveguides, a waveguide coupling element and two output waveguides and the width on the input side of the waveguide coupling element is greater than the total width of the two input waveguides while the width on the output side of the waveguide coupling element is greater than the total width of the two output waveguides, there is an advantage that generation of radiation mode light on the incoming side and the outgoing side of the waveguide coupling element can be suppressed to improve the performance as a device.

[0045] 2. Since the width of the polarization wave separating waveguide on the input side of the waveguide coupling element is greater than the total width of the two input waveguides and the width on the output side of the waveguide coupling element is greater than the total width of the two output waveguides, there is an advantage that generation of radiation mode light on the incoming side and the outgoing side of the waveguide coupling element can be suppressed to improve the performance as a device.

[0046] 3. Since the distance between the two input waveguides in the waveguide coupling element is set to one half or less of the width of the portion at which each of the input waveguides is connected to the waveguide coupling element and the width of the waveguide coupling element on the input waveguides side is set to 2.5 times to 3.5 times the width, there is an advantage that generation of radiation mode light at least on the incoming side of the waveguide coupling element can be suppressed to improve the performance as a device.

[0047] 4. Since the distance between the two output waveguides in the waveguide coupling element is set to one half or less of the width of the portion at which each of the output waveguides is connected to the waveguide coupling element and the width of the waveguide coupling element on the output waveguides side is set to 2.5 times to 3.5 times the width, there is an advantage that generation of radiation mode light at least on the outgoing side of the waveguide coupling element can be suppressed to improve the performance as a device.

[0048] 5. Since generation of radiation mode light can be suppressed by the polarization beam splitter, where polarization beam splitters are connected at multiple stages particularly as in an acousto-optic tunable filter apparatus, the extinction ratio of the polarization beam splitters connected in multiple stages can be improved.

[0049] The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference characters.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050]FIG. 1 is a schematic view showing an optical waveguide device according to a first embodiment of the present invention;

[0051]FIG. 2 is a waveform diagram illustrating a length of a waveguide coupling element with which the optical waveguide device in the first embodiment can be used as a 3 dB coupler;

[0052]FIG. 3 is a schematic top plan view showing the optical waveguide device in the first embodiment when it functions as a polarization beam splitter;

[0053]FIG. 4(a) is a schematic view showing a manner of interference in a waveguide coupling element of the optical waveguide device according to the first embodiment, and FIG. 4(b) is a schematic view showing a manner of interference in a waveguide coupling element of an optical multiplexer/demultiplexer shown in FIG. 16;

[0054]FIG. 5(a) is a schematic view showing a manner of interference in a waveguide coupling element of the optical waveguide device according to the first embodiment, and FIG. 5(b) is a schematic view showing a manner of interference in a waveguide coupling element of an optical multiplexer/demultiplexer shown in FIG. 16;

[0055] FIGS. 6 to 9 are schematic views showing optical devices according to modifications to the first embodiment of the present invention;

[0056]FIG. 10 is a schematic view showing an optical modulator according to a second embodiment of the present invention;

[0057]FIGS. 11 and 12 are views illustrating operation and effects of the second embodiment of the present invention;

[0058]FIG. 13 is a schematic view showing an optical modulator according to a modification to the second embodiment of the present invention;

[0059]FIG. 14 is a block diagram showing an optical modulator according to a third embodiment of the present invention;

[0060]FIG. 15 is a block diagram showing an optical modulator according to a fourth embodiment of the present invention;

[0061]FIG. 16 is a schematic view showing a conventional optical multiplexer/demultiplexer having two inputs and two outputs;

[0062]FIG. 17 is a waveform diagram illustrating a fact that, by adjusting the length of a waveguide coupling element, a characteristic of the optical multiplexer/demultiplexer shown in FIG. 16 can be varied;

[0063]FIG. 18 is a schematic view showing an example of a case wherein the optical multiplexer/demultiplexer shown in FIG. 16 is applied to a Mach-Zehnder type modulator; and

[0064]FIG. 19 is a waveform diagram illustrating a subject to be resolved by the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0065] In the following, embodiments of the present invention are described with reference to the drawings.

[0066] Description of the First Embodiment

[0067]FIG. 1 is a schematic top plan view showing an optical waveguide device 1 according to a first embodiment of the present invention. The optical waveguide device 1 shown in FIG. 1 includes a substrate 2 having an electro-optical effect and an optical waveguide 10 formed on the substrate 2. The optical waveguide 10 includes two input waveguides 11 and 12, two output waveguides 13 and 14, and a waveguide coupling element 15.

[0068] Here, the input waveguides 11 and 12 propagate lights incoming to the optical waveguide device 1, and the waveguide coupling element 15 is connected to the two input waveguides 11 and 12 with a distance d₁ therebetween and causes the lights propagating in the two input waveguides 11 and 12 to interfere with each other. Further, the output waveguides 13 and 14 are branched with another distance d₂ from the waveguide coupling element 15 and propagate the lights caused to interfere with each other by the waveguide coupling element 15.

[0069] Further, the waveguide coupling element 15 has a characteristic width w_(w) of the present invention (that is, the width w_(w1) on the input waveguides 11 and 12 side and the width w_(w2) on the output waveguides 13 and 14 side) and the length Lc thereof is designed so that light inputted from one of the input waveguides 11 and 12 is branched with a predetermined branching ratio to the output waveguides 13 and 14.

[0070] It is to be noted that the two input waveguides 11 and 12 of the optical waveguide device 1 according to the first embodiment are formed with a uniform width w₀₁, and are connected to the waveguide coupling element 15 with an opening angle of 0.5 degrees or more at portions at which the two input waveguides 11 and 12 and the waveguide coupling element 15 are connected (coupled) to each other. Similarly, also the output waveguides 13 and 14 are formed with a uniform width w₀₂ (≈ w₀₁) substantially equal to that of the input waveguides 11 and 12, and are connected to the waveguide coupling element 15 with an opening angle θ of 0.5 degrees or more (for example, 0.5 degrees) at portions at which the two output waveguides 13 and 14 and the waveguide coupling element 15 are connected (coupled) to each other.

[0071] Consequently, the distances between the waveguides 11 to 14 are secured to facilitate connection where the input and output waveguides 11 to 14 of the optical waveguide device 1 are connected to, for example, optical fibers or the like.

[0072] Incidentally, also in the optical waveguide device 1, by adjusting the length Lc of the waveguide coupling element 15 similarly as in the optical multiplexer/demultiplexer 700 described hereinabove with reference to FIG. 16, when light (P₁+P₂) is inputted to one of the two input waveguides 11 and 12, if lights P₁ and P₂ are branched to the output waveguides 13 and 14 as shown in FIG. 17, respectively, then the branching ratio P₁/(P₁+P₂) of the input light to be inputted to the output waveguides 13 and 14 can be varied.

[0073] For example, where the length Lc of the waveguide coupling element 15 is Lc=L₁, light is propagated only to the output waveguide 13 [P₁/(P₁+P₂)=1], but where the length Lc is Lc=L₃, light is propagated only to the output waveguide 14 [P₁/(P₁+P₂)=0], and further, where the length Lc is Lc=L₂, light is propagated to both of the output waveguides 13 and 14 with a branching ratio of 1:1 [P₁/(P₁+P₂)=0.5].

[0074] In other words, if the length of the waveguide coupling element 15 is L₂ shown in FIG. 2, then the optical waveguide device 1 can be used as a 3 dB coupler.

[0075] Further, where the length L₃ of the waveguide coupling element 15 with which, when light inputted to the input waveguide 11 is TE polarized light, the light is outputted only to the output waveguide 14 is represented by L3TE and the length L₁ of the waveguide coupling element 15 with which, when TM polarized light is inputted to the input waveguide 11, the light is outputted only to the output waveguide 13 is represented by L1TM, if the opening angle θ of the input and output waveguides 11 to 14 and waveguide production conditions such as the waveguide widths w₀₁ and w₀₂ of the input and output waveguides 11 to 14 and so forth are adjusted, then the length Lc can be set to Lc=L₂(=L3TE=L1TM), for example, as shown in FIG. 2.

[0076] In other words, if the length of the waveguide coupling element 15 is L₂ shown in FIG. 2, then, the optical waveguide device 1 can used to function, for example, as shown in FIG. 3, as a polarization beam splitter (polarization coupler) which polarizes and splits light inputted from the input waveguide 11 so that TM polarized light and TE polarized light can be outputted from the output waveguides 13 and 14, respectively.

[0077] Accordingly, the length Lc of the waveguide coupling element 15 can be set so that, where a polarized wave of one of the two polarization modes of TM and TE is inputted from one of the two input waveguides 11 and 12, the polarized wave described above may be branched with a desired branching ratio to the two output waveguides 13 and 14. Further, the length Lc of the waveguide coupling element 15 can be set so that, where polarized waves of the two polarization modes of TM and TE are inputted from one of the two input waveguides 11 and 12, the polarized waves described above may be branched with a desired branching ratio to the two output waveguides 13 and 14.

[0078] The characteristic width w_(w) of the waveguide coupling element 15 of the optical waveguide device 1 according to the present invention is described in detail below.

[0079] While the width of the waveguide coupling element 706 in the optical multiplexer/demultiplexer 700 shown in FIG. 16 is substantially equal to the width as taken between the opposite outer sides of the two input waveguides 702 and 703 or the two output waveguides 704 and 705, in the optical waveguide device 1 according to the first embodiment, the waveguide coupling element 15 has a shape wherein it projects in the opposite widthwise directions.

[0080] In particular, in the optical waveguide device 1, the width w_(w1) on the input side of the waveguide coupling element 15 is greater than the total width 2w₀₁ of the two input waveguides 11 and 12, and the width w_(w2) on the output side of the waveguide coupling element 15 is greater than the total width 2w₀₂ of the two output waveguides 13 and 14. Consequently, appearance of radiation mode light when the lights from the input waveguides 11 and 12 interfere with each other in the waveguide coupling element 15 and appearance of radiation mode light when light propagated in the waveguide coupling element 15 is branched to the output waveguides 13 and 14 can be suppressed.

[0081] In particular, in the optical waveguide device 1 according to the first embodiment, the width w_(w1) of the waveguide coupling element on the input waveguides 11 and 12 side is greater than the sum 2w₀₁+d₁ of the sum 2w₀₁ of the widths of the portions at which the two input waveguides 11 and 12 are connected to the waveguide coupling element 15 and the distance d₁ so that the radiation mode light can be suppressed better than where the width w_(w1) is equal to the sum 2w₀₁+d₁.

[0082] Similarly, the width w_(w2) of the waveguide coupling element 15 on the output side waveguides 13 and 14 side is greater than the sum 2w₀₂+d₂ of the sum 2w₀₂ of the widths of the portions at which the two output waveguides 13 and 14 are connected to the waveguide coupling element 15 and the distance d₂ so that radiation mode light can be suppressed better than where the width w_(w2) is equal to the sum 2w₀₂+d₂.

[0083] It is to be noted that, while the input and output waveguides 11 to 14 described above are connected to the waveguide coupling element 15 with an angle of 0.5 degrees, the connection angle θ of the optical waveguides 11 to 14 to the interference waveguide 15 is sufficiently small, and the widths of portions at which the waveguides 11 to 14 are connected to the waveguide coupling element 15 can be approximated to the waveguide widths w₀₁ and w₀₂. Accordingly, the sum of the widths of the portions at which the two input waveguides 11 and 12 described above are connected to the waveguide coupling element 15 can be approximated to 2w₀₁, and the sum of the widths of the portions at which the output waveguides 13 and 14 are connected to the waveguide coupling element 15 can be approximated to 2w₀₂.

[0084] At this time, where the distance d₁ between the connection portions between the input waveguides 11 and 12 and the waveguide coupling element 15 is one half or less of the width w₀₁ of the input waveguides 11 and 12, it is desirable that the waveguide coupling element 15 has the width w_(w1) widened by substantially one half of the width w₀₁ of the input waveguides 11 and 12 from the outside edges of the input waveguides 11 and 12 at the connection location.

[0085] Similarly, where the distance d₂ between the connection portions between the output waveguides 13 and 14 and the waveguide coupling element 15 is one half or less of the width w₀₂ of the output waveguides 13 and 14, it is desirable that the waveguide coupling element 15 has the width w_(w2) widened by substantially one half of the width w₀₂ of the output waveguides 11 and 12 from the outside edges of the output waveguides 13 and 14 at the connected location.

[0086] In this instance, the distance d₁ on the waveguide coupling element 15 is one half or less of the width w₀₁ of the portions described above at which the input waveguides are connected to the waveguide coupling element, and the width w_(w1) of the waveguide coupling element 15 on the output waveguides 11 and 12 side is 2.5 times to 3.5 times the width w₀₁ just described.

[0087] Similarly, the distance d₂ on the waveguide coupling element 15 is one half or less of the width w₀₂ of the portions at which the output waveguides 13 and 14 are connected to the waveguide coupling element 15, and the width w_(w2) of the waveguide coupling element 15 on the output waveguides 13 and 14 side is 2.5 times to 3.5 times the width w₀₂ described above.

[0088] Further, in the optical waveguide device 1 according to the first embodiment, the distances d₁ and d₂ are substantially equal to each other, and the width w₀₁ of the input waveguides 11 and 12 and the width w₀₂ of the output waveguides 13 and 14 are substantially equal to each other. Furthermore, the widths w_(w1) and w_(w2) of the waveguide coupling element 15 on the input and output waveguides 11 to 14 sides are substantially equal to each other.

[0089] For example, the widths of the input and output waveguides 11 to 14 are w₀₁=w₀₂=approximately 7 μm, and the distances are d₁=d₂=approximately 1 μm, and end portions in widthwise directions of the waveguide coupling element 15 project by approximately 2.5 μm from the outer edge portions of the input and output waveguides 11 to 14.

[0090] By the configuration described above, in the optical waveguide device 1 according to the first embodiment of the present invention, when light propagating in the input waveguide 11 or 12 is inputted to the waveguide coupling element 15 and when light propagating in the waveguide coupling element 15 is branched to the output waveguides 13 and 14, appearance of radiation mode light (radiation light) can be suppressed as described below.

[0091]FIG. 4(a) is a schematic view illustrating a manner of interference in the waveguide coupling element 15 where lights P₀₁ and P₀₂ having the same phase are inputted to the input waveguides 11 and 12 of the optical waveguide device 1 according to the first embodiment, respectively, and FIG. 4(b) is a schematic view illustrating a manner of interference in the waveguide coupling element 706 where such lights P₀₁ and P₀₂ are inputted to the input waveguides 702 and 703 of the optical multiplexer/demultiplexer 700 shown in FIG. 16, respectively.

[0092] Meanwhile, FIG. 5(a) is a schematic view illustrating a manner of interference in the waveguide coupling element 15 where lights P₀₃ and P₀₄ having the opposite phases to each other are inputted to the input waveguides 11 and 12 of the optical waveguide device 1 according to the first embodiment, respectively, and FIG. 5(b) is a schematic view illustrating a manner of interference in the waveguide coupling element 706 where such lights P₀₃ and P₀₄ are inputted to the input waveguides 702 and 703 of the optical multiplexer/demultiplexer 700 shown in FIG. 16, respectively.

[0093] By combinations including the phase relations where light inputs to the two input waveguides 11 and 12 are the same phase inputs and they are the opposite phase inputs seen in FIGS. 4(a) and 5(a) [or FIGS. 4(b) and 5(b)], all light input methods to the two input waveguides 11 and 12 can be represented. Here, where the light inputs are the same phase inputs, 0th-order mode light is excited in the waveguide coupling element 15 at the center, but where the light inputs are the opposite phase inputs, first-order mode light is excited in the waveguide coupling element 15.

[0094] Here, as shown in FIGS. 5(a) and 5(b), in both of the optical waveguide device 1 according to the present embodiment and the conventional optical multiplexer/demultiplexer 700, where lights are inputted with the opposite phases to each other to the two input waveguides 11 and 12 (in the optical multiplexer/demultiplexer 700, to the input waveguides 702 and 703), the mode of light is smoothly changed from the input waveguide to the waveguide coupling element.

[0095] In contrast, where lights are inputted with the same phases as each other to the two input waveguides 11 and 12 (in the optical multiplexer/demultiplexer 700, to the input waveguides 702 and 703), as seen from FIGS. 4(a) and 4(b), radiation light appears only in the optical multiplexer/demultiplexer 700.

[0096] In particular, at a connection section B1 of the input waveguides 702 and 703 and the waveguide coupling element 706, the shape 800 of the light mode shape has two peaks with regard to the input waveguides 702 and 703, but the shape 900 of the light mode has one peak with regard to the waveguide coupling element 706. Therefore, a mode mismatch [refer to a slanting line area in the connection section B1 of FIG. 4(b)] occurs and radiation light 910 is generated.

[0097] Similarly, at a connection section B2 of the waveguide coupling element 706 and the output waveguides 703 and 704, the shape 801 of the light mode has two peaks with regard to the output waveguides 703 and 704, but the shape 901 of the light mode has one peak with regard to the waveguide coupling element 706. Therefore, a mode mismatch [refer to a slanting line area in the connection section B2 of FIG. 4(b)] occurs and radiation light 920 is generated.

[0098] In the waveguide structure shown in FIG. 4(b), the degree of the mode mismatch is high, and loss by generation of the radiation lights 910 and 920 occurs. However, if the width of the waveguide coupling element 15 is widened as in the structure of the optical waveguide device 1 in the first embodiment shown in FIG. 4(a), then the electric field distribution of the 0th-order mode light is widened, and consequently, the degree of the mode mismatch at each of the connection sections B1 and B2 is decreased [refer to slanting line areas in the connection sections B1 and B2 of FIG. 4(a)]. Consequently, loss by generation of radiation light is suppressed significantly.

[0099] In particular, if the width w_(w1) of the waveguide coupling element 15 on the input waveguides 11 and 12 side is set to a value approximately 2.5 times to 3.5 times the width w₀₁ of the input waveguides 11 and 12, then the electric field distribution of 0th-order mode light in the connection section B1 can be widened effectively, and consequently, the degree of the mode mismatch can be suppressed low.

[0100] Further, if the width w_(w2) of the waveguide coupling element 15 on the output waveguides 13 and 14 side is set to a value approximately 2.5 times to 3.5 times the width w₀₂ of the output waveguides 13 and 14, then the electric field distribution of 0th-order mode light in the connection section B2 can be widened effectively, and consequently, the degree of the mode mismatch can be suppressed low.

[0101] Further, by varying the length Lc of the waveguide coupling element 15 described above, the phase difference φ between the 0th-order mode light and the 1st-order mode light excited as described above can be varied, and as a result, the branching ratio of the output waveguides 13 and 14 can be varied. For example, if the phase difference Ψo between the same phase input and the opposite phase input is 0 degree, that is, the inputs have the same phase and the phase difference φ between the 0th-order mode light and the 1st-order mode light in the waveguide coupling element 15 is 0 degree, that is, the lights have the same phase and then they are added to each other, then the input of light is only light P₀₁ from the input waveguide 11, and the output of light is only light P₁₁ from the output waveguide 13.

[0102] Further, where the phase difference Ψo between the same phase input and the opposite phase input is Ψo=0 degree and the phase difference φ between the 0th-order mode light and the 1st-order mode light in the waveguide coupling element 15 is φ=90 degree, the light inputted from the input waveguide 11 is outputted with a branching ratio of 1:1 to the two output waveguides 13 and 14.

[0103] In this manner, with the optical waveguide device 1 according to the first embodiment of the present invention, since the width on the input side of the waveguide coupling element 15 is greater than that of the two input waveguides 11 and 12 and the width on the output side of the waveguide coupling element 15 is greater than that of the two output waveguides 13 and 14, there is an advantage that appearance of radiation mode light on the incoming and outgoing sides in the waveguide coupling element 15 is suppressed and performances as a device can be improved.

[0104] It is to be noted that, in the optical waveguide device 1 according to the first embodiment described above, the distance d₁ in the waveguide coupling element 15 is set to one half or less of the width w₀₁ of the portion at which each of the input waveguides 11 and 12 is connected to the waveguide coupling element 15 and the width w_(w1) of the waveguide coupling element 15 on the input waveguides 11 and 12 side is set to 2.5 times to 3.5 times the width w₀₁ just described, and further, the distance d₂ in the waveguide coupling element 15 is set to one half or less of the width w₀₂ at the portion at which each of the output waveguides 13 and 14 is connected to the waveguide coupling element 15 and the width w_(w2) of the waveguide coupling element 15 on the output waveguides 13 and 14 side is set to 2.5 times to 3.5 times the width w₀₂ just described. However, the present invention is not limited to this, but also the characteristic width of the present invention can be given only to the width of the waveguide coupling element 15 on the input waveguides 11 and 12 side or only to the width of the waveguide coupling element 15 on the output waveguides 13 and 14 side.

[0105] In particular, if the width of the waveguide coupling element 15 on the input waveguides 11 and 12 side has the characteristic width of the present invention, then generation of radiation mode light at least on the incoming side in the waveguide coupling element 15 can be suppressed. Meanwhile, if the width of the waveguide coupling element 15 on the output waveguides 13 and 14 side has the characteristic width of the present invention, then generation of radiation mode light at least on the outgoing side in the waveguide coupling element 15 can be suppressed.

[0106] Further, in the optical waveguide device 1 according to the first embodiment described above, while the distances d₁ and d₂, the widths w₀₁ and w₀₂, and the widths w_(w1) and w_(w2) are substantially equal to each other, according to the present invention, the scales just described can be set so as to differ from each other. Also in the case wherein the characteristic width of the present invention is given to the width of the waveguide coupling element 15 only on the input waveguides 11 and 12 side or to the width of the waveguide coupling element 15 only on the output waveguides 13 and 14 side, the scales just described can be set so as to differ from each other similarly as in the example described above.

[0107] For example, also it is possible to configure the optical waveguide device such that, as in an optical waveguide device 1A shown in FIG. 6, the width w₀₂ of output waveguides 13A and 14A is set greater than the width w₀₁ of the input waveguides 11 and 12 (w₀₂>w₀₁) and the characteristic width of the present invention is given only to the width of a waveguide coupling element 15A on the input waveguides 11 and 12 side. In particular, as shown in FIG. 6, the distance d₁ in the waveguide coupling element 15 is set to one half or less of the width w₀₁ of a portion at which each of the input waveguides 11 and 12 is connected to the waveguide coupling element 15A, and the width w_(w1) of the waveguide coupling element 15A on the input waveguides 11 and 12 side is set to 2.5 times to 3.5 times the width w₀₁ described above.

[0108] Further, also it is possible to configure the optical waveguide device such that, as in an optical waveguide device 1B shown in FIG. 7, the width w₀₁ of input waveguides 11B and 12B is set greater than the width w₀₂ of the output waveguides 13 and 14 (w₀₁>w₀₂), and the characteristic width of the present invention is given only to the width of a waveguide coupling element 15B on the output waveguides 13 and 14 side. In particular, the distance d₂ in the waveguide couple 15B is set to one half or less of the width w₀₂ of a portion at which each of the output waveguides 13 and 14 is connected to the waveguide coupling element 15B, and the width w_(w2) of the waveguide coupling element 15B on the output waveguides 13 and 14 side is set to 2.5 times to 3.5 times the width w₀₂ described above.

[0109] Further, in the optical waveguide device 1 according to the first embodiment described above and the optical waveguide devices 1A and 1B shown in FIGS. 6 and 7, the waveguide coupling elements 15, 15A and 15B have a rectangular pattern having a fixed width (w_(w1)=w_(w2)). However, according to the present invention, also a trapezoidal pattern having a width (w_(w1)≠w_(w2)) different between the input and output waveguides can be given to the waveguide coupling elements 15, 15A and 15B.

[0110] For example, also it is possible to configure the optical waveguide device such that, as in an optical waveguide device 1C shown in FIG. 8, the width w₀₁ of input waveguides 11C and 12C is set greater than the width w₀₂ of output waveguides 13C and 14C (w₀₁>w₀₂), and the characteristic width of the present invention is given to the width w_(w1) of a waveguide coupling element 15C on the input waveguides 11C and 12C side and the width w_(w2) of the waveguide coupling element 15B on the output waveguides 13 and 14 side while the widths are different from each other.

[0111] Further, also it is possible to configure the optical waveguide device such that, as in an optical waveguide device 1D shown in FIG. 9, the width w₀₁ of input waveguides 11D and 12D is set smaller than the width w₀₂ of output waveguides 13D and 14D (w₀₁<w₀₂), and the characteristic width of the present invention is given to the width w_(w1) of a waveguide coupling element 15D on the input waveguides 11D and 12D side and the width w_(w2) of the waveguide coupling element 15B on the output waveguides 13 and 14 side while the widths are different from each other.

[0112] It is to be noted that, in the case of FIG. 8 (or FIG. 9) described above, the distance d₁ in the waveguide coupling element 15C (15D) is set to one half or less of the width w₀₁ of the portion at which each of the input waveguides 11C and 12C (11D and 12D) is connected to the waveguide coupling element 15C (15D), and the width w_(w1) of the waveguide coupling element 15C (15D) on the input waveguides 11C and 12C (11D and 12D) side is set to 2.5 times to 3.5 times the width w₀₁ described above. Further, the distance d₂ in the waveguide coupling element 15C (15D) is set to one half or less of the width w₀₂ of the portion at which each of the output waveguides 13 c and 14C (13D and 14D) is connected to the waveguide coupling element 15C (15D), and the width w_(w2) of the waveguide coupling element 15C (15D) on the output waveguides 13C and 14C (13D and 14D) side is set to 2.5 times to 3.5 times the width w₀₂ described above.

[0113] Further, in the optical waveguide device 1 according to the first embodiment described above, the input and output waveguides 11 to 14 are connected to the waveguide coupling element 15 with the opening angle θ of 0.5 degrees or more at the portions at which the input and output waveguides 11 to 14 are connected to the waveguide coupling element 15. However, according to the present invention, even if the opening angle just described is not given to the input and output waveguides 11 to 14, generation of radiation light at least on the incoming and outgoing sides of the waveguide coupling element 15 can be suppressed.

[0114] [B] Description of the Second Embodiment

[0115]FIG. 10 is a schematic view showing an optical modulator according to a second embodiment of the present invention. In the optical modulator 100 shown in FIG. 10, reference numeral 101 denotes a substrate having an electro-optical effect and formed from lithium niobate or the like, and the substrate may have, for example, a width of 1 to 2 mm and a length of 80 to 90 mm in a longitudinal direction.

[0116] Further, reference numeral 110 denotes a Mach-Zehnder type optical waveguide mounted on the substrate 101. The Mach-Zehnder type optical waveguide 110 branches input light into two lights with a ratio of 1:1 and multiplexes the light modulated by a signal electrode section 120 and a bias electrode section 130 to output multiplexed light as modulated signal light, and includes a branching waveguide 111, linear waveguides 112 and 113 and a multiplexing waveguide 114.

[0117] Further, the branching waveguide 111 of the Mach-Zehnder type optical waveguide 110 branches inputted propagation light into two lights. The two linear waveguides 112 and 113 propagate the two propagation lights branched by the branching waveguide 111. The multiplexing waveguide 114 multiplexes the propagation lights from the two linear waveguides 112 and 113. Further, a 3 dB coupler 111 a is formed on the branching waveguide 111, and a 3 dB coupler 114 a and an X dB coupler 114 b are formed on the multiplexing waveguide 114.

[0118] Here, the 3 dB coupler 111 a branches the input light to the linear waveguides 112 and 113 with a ratio of 1:1. The coupler 114 a multiplexes the lights propagating in the linear waveguides 112 and 113 and outputs the multiplexed light as a pair of complementary signals. The coupler 114 a outputs one of the complementary signals as signal light and the other complementary signal as monitor light. Further, the coupler 114 b further branches the monitor light branched by the coupler 114 a and emits one of the branched lights to a monitor photodiode 150 side, and functions as an attenuator which attenuates the light intensity in order to supply monitor light to the monitor photodiode 150.

[0119] It is to be noted that the ratio (X dB) with which the light is attenuated at the coupler 114 b is suitably adjusted as the light intensity to be supplied to the monitor photodiode 150.

[0120] Accordingly, the couplers 114 a and 114 b described above function as a coupler mounted on the outgoing side of the Mach-Zehnder type optical waveguide 110 for fetching the monitor light.

[0121] It is to be noted that the couplers 111 a, 114 a and 114 b have a characteristic configuration according to the present invention hereinafter described.

[0122] Further, the signal electrode section 120 supplies a voltage from a data (DATA) signal supplying source 170 to the linear waveguides 112 and 113 and includes a signal electrode 121 and a ground electrode 140 connected to the data signal supplying source 170. In particular, an electric field of the linear waveguides 112 and 113 is varied by a voltage variation between the signal electrode 121 and the ground electrode 140 just described to modulate the lights which propagate in the linear waveguides 112 and 113.

[0123] Further, the bias electrode section 130 is mounted on the optical waveguide 110 in order to supply a bias voltage. In particular, the bias electrode 130 supplies a voltage from a DC (Direct Current) control section 160 to the linear waveguides 113 and 112 and includes a bias electrode 131 connected to the DC control section 160 and the ground electrode 140 for common use by the signal electrode section 120.

[0124] Further, the monitor photodiode 150 detects a light signal for monitoring fetched by the couplers 114 a and 114 b described above. The DC control section 160 functions as a bias voltage control section for controlling a bias voltage to be supplied to the bias electrode section 130 based on the light signal detected by the monitor photodiode 150.

[0125] Next, configurations of the couplers 111 a, 114 a and 114 b are described in detail below.

[0126] All of the couplers 111 a, 114 a and 114 b have, similarly to the optical waveguide device 1 in the first embodiment described above, the characteristic width according to the present invention. Here, taking notice of the coupler 114 a, it includes two input waveguides 114 a-1 and 114 a-2, a waveguide coupling element 114 a-5, and output couplers 114 a-3 and 114 a-4.

[0127] Here, each of the two input waveguides 114 a-1 and 114 a-2 receives light outputted from one of the linear waveguides 112 and 113 of the Mach-Zehnder type optical waveguide 110. In the optical modulator 100 according to the second embodiment, the input waveguide 114 a-1 receives the light outputted from the linear waveguide 112, and the input waveguide 114 a-2 receives the light outputted from the linear waveguide 113.

[0128] Further, the waveguide coupling element 114 a-5 couples the two input waveguides 114 a-1 and 114 a-2 with a distance d₁ so that the lights propagated in the two input waveguides may interfere with each other. The output waveguides 114 a-3 and 114 a-4 are branched from the waveguide coupling element 114 a-5 with another distance d₂. The output waveguide 114 a-3 is used as an output waveguide for propagating the signal light, and the output waveguide 114 a-4 is used as a monitoring output waveguide.

[0129] Here, the width on the input side of the waveguide coupling element 114 a-5 is greater than the total width of the two input waveguides 114 a-1 and 114 a-2, and the width on the output side of the waveguide coupling element 114 a-5 is greater than the total width of the two output waveguides 114 a-3 and 114 a-4.

[0130] Further, the distance d₁ in the waveguide coupling element 114 a-5 is set to one half or less of the width w₀₁ of a portion at which the input waveguide 114 a-1 (or the input waveguide 114 a-2) is connected to the waveguide coupling element 114 a-5, and the width w_(w1) of the waveguide coupling element on the input waveguides 114 a-1 and 114 a-2 side is set so as to have a width of 2.5 times to 3.5 times the width w₀₁.

[0131] Similarly, the distance d₂ in the waveguide coupling element 114 a-5 is set to one half or less of the width w₀₂ of a portion at which the output waveguide 114 a-3 (or the output waveguide 114 a-4) is connected to the waveguide coupling element 114 a-5, and the width w_(w2) of the waveguide coupling element on the output waveguides 114 a-3 and 114 a-4 side is set so as to have a width of 2.5 times to 3.5 times the width w₀₂.

[0132] Consequently, particularly in the 3 dB coupler 114 a, similarly as in the case of the first embodiment described above, generation of radiation mode light when the lights from the linear waveguides 112 and 113 interfere with each other at the waveguide coupling element 114 a-5 can be suppressed. Therefore, as hereinafter described, incoincidence between the voltage value V₁ when the signal light is in an on-state and the voltage value V₂ when the monitor light is in an off-state can be improved.

[0133] Further, in the 3 dB couplers 111 a and 114 a described above, the length Lc of the waveguide coupling element (for the case of the coupler 114 a, refer to reference character 114 a-5) is set so as to be equal to the length L₂ shown in FIG. 17 described above. In particular, the length of the waveguide coupling element (114 a-5) is set so that the light which propagates in one of the two input waveguides (similarly refer to reference characters 114 a-1 and 114 a-2) is branched with a power ratio of 1:1 to the two output waveguides (similarly refer to reference characters 114 a-3 and 114 a-4).

[0134] Consequently, particularly in the 3 dB coupler 114 a, the output waveguide 114 a-3 from between the two output waveguides can be used as a waveguide for propagating the signal light, and the other output waveguide 114 a-4 from between the two output waveguide can be used as a monitoring waveguide.

[0135] It is to be noted that the coupler 114 b further branches the light fetched as monitor light by the 3 dB coupler 114 a and supplies one of the branched lights to the monitor photodiode 150. Further, the length of the waveguide coupling element in the coupler 114 b is set so that the branching ratio described above may be a branching ratio with which the branched light can be received with an optimum sensitivity by the monitor photodiode 150 in the next stage.

[0136] Further, also in the couplers 111 a, 114 a and 114 b of the optical modulator 100 according to the present embodiment, the distances d₁ and d₂ described above are set substantially equal to each other, and the widths w₀₁ and w₀₂, and besides the widths w_(w1) and w_(w2) of the waveguide coupling element, are set substantially equal to each other. In addition, the input waveguides and the output waveguides are connected to the waveguide coupling element with an opening angle of θ=0.5 degrees or more at portions at which they are connected to the waveguide coupling element.

[0137] By the configuration described above, in the optical modulator 100 according to the second embodiment of the present invention, a voltage of a data signal from the data signal supplying source 170 is applied to the signal electrode section 120 to modulate light which propagates in the Mach-Zehnder type optical waveguide 110 so that modulated light is outputted.

[0138] At this time, part of the modulated light fetched by the couplers 114 a and 114 b is received as monitor light by the monitor photodiode 150, and a bias voltage corresponding to the reception light signal is applied to the bias electrode section 130 through the DC control section 160. Consequently, operation of the modulated signal light is controlled.

[0139] Further, if, in the 3 dB coupler 114 a on the outgoing side of the Mach-Zehnder type optical waveguide 110, the width on the incoming side (input waveguides 114 a-1 and 114 a-2 side) is set to the width w_(w1) described above and the width on the outgoing side (output waveguides 114 a-3 and 114 a-4 side) is set to the width w_(w2) described above, then generation of radiation mode light when the lights from the linear waveguides 112 and 113 interfere with each other at the waveguide coupling element 114 a-5 can be suppressed. Consequently, as shown in FIG. 11, incoincidence between the voltage value V₁ when the signal light is in an on-state and the voltage value V₂ when the monitor light is in an off-state, that is, incoincidence of operating points, is eliminated.

[0140] If, for example, the widths of the input and output waveguides 114 a-1 to 114 a-4 are set to w₀₁=w₀₂=7 μm and the distances are set to d₁=d₂=1 μm, and further, an end portion in widthwise directions of the waveguide coupling element 114 a-5 projects by approximately 2.5 μm from each of the outer edge portions of the input and output waveguides 114 a-1 to 114 a-4, then the width of the waveguide coupling element 114 a-5 can be set to approximately 20 μm. With the configuration just described, the operating point displacement can be stably reduced independently of a device error, for example, as shown in FIG. 12.

[0141] It is to be noted that FIG. 12 is a view for evaluation of a performance against the operating point displacement based on a plurality of prototype devices produced for different waveguide coupling element widths w_(w), and four different plot patterns ([⋄], [▴], [□], []) denote the different operating point displacements of the different devices. As shown in FIG. 12, if the widths w_(w) of the waveguide coupling element 114 a-5 are set to 19 to 20 μm, then device errors, that is, unique errors between devices when a plurality of devices are produced in the same conditions, can be suppressed to stably reduce the operating point displacements.

[0142] In this manner, with the optical modulator according to second embodiment of the present invention, since the 3 dB coupler 114 a on the outgoing side of the Mach-Zehnder type optical waveguide 110 is configured such that the width of the incoming side (input waveguides 114 a-1 and 114 a-2 side) is set as the width w_(w1) and the width of the outgoing side (output waveguides 114 a-3 and 114 a-4 side) is set as the width w_(w2), generation of radiation mode light when the lights from the linear waveguides 112 and 113 interfere with each other at the waveguide coupling element 114 a-5 can be suppressed. Consequently, there is an advantage that incoincidence between the operating points can be suppressed.

[0143] It is to be noted that, in the optical modulator 100 according to the second embodiment described above, the widths w_(w1) and w_(w2) of the incoming and outgoing sides of the waveguide coupling element in the three couplers 111 a, 114 a and 114 b have the characteristic width of the present invention. However, the present invention is not limited to this, but the characteristic width of the present invention can be given, for example, only to the width of the waveguide coupling element on the input waveguides side or only to the width of the waveguide coupling element on the output waveguides side.

[0144] Further, in the three couplers 111 a, 114 a and 114 b, while the distances d₁ and d₂ and the widths w₀₁ and w₀₂ and besides the widths w_(w1) and w_(w2) are substantially equal to each other, according to the present invention, they can be configured otherwise such that the scales of them suitably differ from each other. Also where the characteristic width of the present invention is given only to the width of the waveguide coupling element on the input waveguides side or only to the width of the waveguide coupling element on the output waveguides side, similarly as in the example just described, the scales described above can be set different from each other.

[0145] In this instance, at least the waveguide coupling element 114 a-5 of the 3 dB coupler 114 a as a synthesis waveguide can be configured such that the characteristic scale of the present invention is given to only the width w_(w1) such that the width w_(w1) is set to 2.5 times to 3.5 times the width w₀₁. Otherwise, the characteristic scale can be given only to the width w_(w2) such that the width w₀₂ is set to 2.5 times to 3.5 times the width w₀₂.

[0146] Further, in the three couplers 111 a, 114 a and 114 b, the input waveguides and output waveguides are connected to the waveguide coupling element with an opening angle θ of 0.5 degrees or more at the connecting portions. However, according to the present invention, even if such an opening angle as just described is not given, generation of radiation light at least on the incoming or outgoing side in the waveguide coupling element can be suppressed.

[0147] Further, in the optical modulator 100 according to the second embodiment described above, while the coupler 114 b which functions as an attenuator is provided, according to the present invention, the optical modulator 100 may be configured otherwise such that the coupler 114 b is omitted but the monitor light fetched by the coupler 114 a is inputted to the monitor photodiode 150 using some other known attenuation element.

[0148] [B1] Modification to the Second Embodiment

[0149]FIG. 13 is a schematic view showing an optical modulator according to a modification to the second embodiment of the present invention. While the optical modulator 200 shown in FIG. 13 is different from that described hereinabove with reference to FIG. 10 in that it includes signal electrode sections 220A and 220B of a dual configuration and includes a bias electrode section 230 of a different configuration, it is basically similar in the other configuration to the optical modulator 100 shown in FIG. 10.

[0150] In particular, also in the optical modulator 200 shown in FIG. 13, a Mach-Zehnder type optical waveguide 210 is formed on a substrate 201 having an electro-optical effect. The Mach-Zehnder type optical waveguide 210 includes a branching waveguide 211, linear waveguides 212 and 213 and a synthesis waveguide 214.

[0151] Here, the branching waveguide 211 of the Mach-Zehnder type optical waveguide 210 branches inputted propagation light into two lights. The two linear waveguides 212 and 213 propagate the propagation lights branched by the branching waveguide 211. The synthesis waveguide 214 synthesizes the propagation lights from the two linear waveguides 212 and 213.

[0152] Further, a 3 dB coupler 211 a is formed on the branching waveguide 211, and another 3 dB coupler 214 a and an X dB coupler 214 b are formed on the synthesis waveguide 214. The 3 dB coupler 211 a branches input light to the linear waveguides 212 and 213 with a branching ratio of 1:1. The couplers 214 a and 214 b function as a coupler provided on the outgoing side of the Mach-Zehnder type optical waveguide 210 for fetching monitor light.

[0153] It is to be noted that the couplers 211 a, 214 a and 214 b are formed, similarly as in those (refer to reference characters 111 a, 114 a and 114 b) shown in FIG. 10 described above, with such a characteristic width according to the present invention as hereinafter described.

[0154] Further, a monitor photodiode 250 detects a light signal of the monitor light fetched by the couplers 214 a and 214 b described above. A DC control section 260 functions as a bias voltage control section for controlling a bias voltage to be supplied to the bias electrode section 230 based on the light signal detected by the monitor photodiode 250.

[0155] Further, the signal electrode sections 220A and 220B of the dual configuration are supplied with data signals having a complementary relationship to each other from complementary data (DATA) signal supplying sources 270A and 270B, respectively. Thus, the signal electrode section 220A modulates light propagating in the linear waveguide 212, and the signal electrode section 220B modulates light propagating in the linear waveguide 213.

[0156] Further, the bias electrode section 230 includes first and second potential supplying electrode sections 231 and 234, first and second waveguide electrode sections 232 and 235 and potential difference producing electrode sections 233 and 236.

[0157] The DC control section 260 described above supplies a voltage for DC control as potential difference information of a negative side potential from a positive side potential. The first potential supplying electrode section 231 of the bias electrode section 230 is supplied with the positive side potential (first potential) just mentioned from the DC control section 260. The second potential supplying electrode section 234 is supplied with the negative side potential just described (second potential) from the DC control section 260.

[0158] Further, the first waveguide electrode section 232 supplies the potential from the first potential supplying electrode section 231 to the linear waveguide 212 from between the two linear waveguides 212 and 213. The second waveguide electrode section 235 supplies the potential from the second potential supplying electrode section 234 to the other linear waveguide 213.

[0159] Further, the two potential difference producing electrode sections 233 are provided in the proximity of the second waveguide electrode section 235 which supplies the potential from the second potential supplying electrode section 234 and supply the potential from the first potential supplying electrode section 231. Similarly, the two potential difference producing electrode sections 236 are provided in the proximity of the first waveguide electrode section 232 which supplies the potential from the first potential supplying electrode section 231 and supply the potential from the second potential supplying electrode section 234.

[0160] Configurations of the couplers 211 a, 214 a and 214 b are described below in detail.

[0161] All of the couplers 211 a, 214 a and 214 b described above have, similarly to the couplers 111 a, 114 a and 114 b in the second embodiment described above, the characteristic width according to the present invention. Here, taking notice of the coupler 214 a, it includes two input waveguides 214 a-1 and 214 a-2, a waveguide coupling element 214 a-5 and two output waveguides 214 a-3 and 214 a-4.

[0162] Here, the two input waveguides 214 a-1 and 214 a-2 are supplied with the light from the Mach-Zehnder type optical waveguide 210. In the optical modulator 200 shown in FIG. 13, the input waveguide 214 a-1 is supplied with the light from the linear waveguide 212, and the input waveguide 214 a-2 is supplied with the light from the linear waveguide 213.

[0163] Further, the waveguide coupling element 214 a-5 couples the two input waveguides 214 a-1 and 214 a-2 with the distance d₁ so that the lights propagating in the two input waveguides may interfere with each other. The output waveguides 214 a-3 and 214 a-4 are branched from the waveguide coupling element 214 a-5 with the distance d₂. The output waveguide 214 a-3 is used as an output waveguide for propagating the signal light, and the output waveguide 214 a-4 is used as a monitoring output waveguide.

[0164] Here, the width on the input side of the waveguide coupling element 214 a-5 is greater than the width of the two input waveguides 214 a-1 and 214 a-2, and the width on the output side of the waveguide coupling element 214 a-5 is greater than the width of the two output waveguides 214 a-3 and 214 a-4.

[0165] Further, the distance d₁ in the waveguide coupling element 214 a-5 is set to one half or less of the width w₀₁ at a portion at which the input waveguide 214 a-1 (or the input waveguide 214 a-2) is connected to the waveguide coupling element 214 a-5, and the width w_(w1) of the waveguide coupling element 214 a-5 on the input waveguides 214 a-1 and 214 a-2 side is set to 2.5 times to 3.5 times the width w₀₁.

[0166] Similarly, the distance d₂ in the waveguide coupling element 214 a-5 is set to one half or less of the width w₀₂ at a portion at which the output waveguide 214 a-3 (or the output waveguide 214 a-4) is connected to the waveguide coupling element 214 a-5, and the width w_(w2) of the waveguide coupling element 214 a-5 on the output waveguides 214 a-1 and 214 a-2 side is set to 2.5 times to 3.5 times the width w₀₂.

[0167] Consequently, particularly in the 3 dB coupler 214 a, similarly as in that in the first embodiment described above, generation of radiation mode light when the lights from the linear waveguides 212 and 213 interfere with each other at the waveguide coupling element 214 a-5 can be suppressed. Therefore, as hereinafter described, the incoincidence between the voltage value V₁ when the signal light is in an on-state and the voltage value V₂ when the monitor light is in an off-state can be suppressed.

[0168] Further, in the 3 dB couplers 211 a and 214 a described above, the length Lc of the waveguide coupling element (for the case of the coupler 214 a, refer to reference character 214 a-5) is set so as to be equal to the length L₂ described hereinabove with reference to FIG. 17. In particular, the length of the waveguide coupling element (214 a-5) is set so that the light which propagates in one of the two input waveguides (similarly refer to reference characters 214 a-1 and 214 a-2) is branched with a power ratio of 1:1 to the two output waveguides (similarly refer to reference characters 214 a-3 and 214 a-4).

[0169] Consequently, particularly in the 3 dB coupler 214 a, the output waveguide 214 a-3 from between the two output waveguides can be used as a waveguide for propagating the signal light, and the other output waveguide 214 a-4 from between the two output waveguides can be used as a monitoring waveguide.

[0170] It is to be noted that the coupler 214 b further branches the light fetched as monitor light by the 3 dB coupler 214 a and supplies one of the branched lights to the monitor photodiode 250. Further, the length of the waveguide coupling element in the coupler 214 b is set so that the branching ratio described above may be a branching ratio with which the branched light can be received with an optimum sensitivity by the monitor photodiode 250 in the next stage.

[0171] Further, also in the couplers 211 a, 214 a and 214 b of the optical modulator 100 shown in FIG. 13, the distances d₁ and d₂ described above are set substantially equal to each other, and the widths w₀₁ and w₀₂, and besides the widths w_(w1) and w_(w2) of the waveguide coupling element, are set substantially equal to each other. Further, the input waveguides and the output waveguides are connected to the waveguide coupling element with an opening angle of θ=0.5 degrees or more at the portions at which they are connected to the waveguide coupling element.

[0172] By the configuration described above, in the optical modulator 200 shown in FIG. 13, the voltages of the complementary data signals from the data signal supplying sources 270A and 270B are applied to the signal electrode sections 220A and 220B, respectively, to modulate the lights propagating in the Mach-Zehnder type optical waveguide 210 so that the modulated lights are outputted.

[0173] At this time, part of the modulated lights fetched by the couplers 214 a and 214 b is received as monitor light by the monitor photodiode 250, and a bias voltage corresponding to the reception light signal is applied to the bias electrode section 230 through the DC control section 160. Consequently, operation of the modulated signal light is controlled.

[0174] Further, since the 3 dB coupler 214 a on the outgoing side of the Mach-Zehnder type optical waveguide 210 is configured such that the width on the incoming side (input waveguides 214 a-1 and 214 a-2 side) is set as the width w_(w1) described above and the width on the outgoing side (output waveguides 214 a-3 and 214 a-4 side) is set as the width w_(w2) described above, generation of radiation mode light when the lights from the linear waveguides 212 and 213 interfere with each other at the waveguide coupling element 214 a-5 can be suppressed. Consequently, similarly as in that in the second embodiment described above, the incoincidence between the voltage value V₁ when the signal light is in an on-state and the voltage value V₂ when the monitor light is in an off-state, that is, incoincidence between the operating points, is suppressed.

[0175] Accordingly, also in the optical modulator 200 shown in FIG. 13, there is an advantage that, since the 3 dB coupler 214 a on the outgoing side of the Mach-Zehnder type optical waveguide 210 is configured such that the width of the incoming side (input waveguides 214 a-1 and 214 a-2 side) is set as the width w_(w1) described above and the width of the outgoing side (output waveguides 214 a-3 and 214 a-4 side) is set as the width w_(w2) described above, generation of radiation mode light when the lights from the linear waveguides 212 and 213 interfere with each other at the waveguide coupling element 214 a-5 can be suppressed, and as a result, the incoincidence between the operating points can be suppressed.

[0176] It is to be noted that also the optical modulator 200 shown in FIG. 13 can be carried out in various modified forms similarly to the optical modulator 100 (refer to FIG. 10) according to the second embodiment described hereinabove.

[0177] [C] Description of the Third Embodiment

[0178]FIG. 14 is a block diagram showing an optical modulator according to a third embodiment of the present invention. In the optical modulator 300 shown in FIG. 14, when compared with the optical modulators 100 and 200 in the second embodiment described above, two Mach-Zehnder type optical waveguides 310 and 320 are formed in series on a single substrate 301 having an electro-optical effect. Further, the Mach-Zehnder type optical waveguide 310 is used for clock modulation, and the other Mach-Zehnder type optical waveguide 320 is used for data modulation.

[0179] Here, in the optical modulator 300 shown in FIG. 14, reference numeral 302 denotes a first signal electrode section. The first signal electrode section 302 is provided on the first Mach-Zehnder type optical waveguide 310 and is supplied with a clock signal, and includes a signal electrode 311 and a ground electrode 313. In particular, a clock signal source 314 is connected to the signal electrode 311 so that an electric signal representative of a clock signal is supplied from the clock signal source 314 to the signal electrode 311.

[0180] Further, reference numeral 304 denotes a second signal electrode section 304. The second signal electrode section 304 is provided on the second Mach-Zehnder type optical waveguide 320 and is supplied with a data signal, and includes a signal electrode 321 and a ground electrode 323. In particular, a data signal producing source 324 is connected to the signal electrode 321 so that an electric signal representative of a data signal is supplied from the data signal producing source 321.

[0181] Further, reference numeral 303 denotes a first bias electrode section. The first bias electrode section 303 is provided on the first Mach-Zehnder type optical waveguide 310 and is supplied with a bias voltage, and includes a bias electrode 312 and a ground electrode 313.

[0182] Further, reference numeral 305 denotes a second bias electrode section. The second bias electrode section 305 is provided on the second Mach-Zehnder type optical waveguide 320 and is supplied with a bias voltage, and includes a bias electrode 322 and a ground electrode 323.

[0183] Further, reference numeral 330 denotes a 3 dB coupler. The 3 dB coupler 330 functions as a branching waveguide on the incoming side which forms the Mach-Zehnder type optical waveguide 310 and branches input light to two linear waveguides, which form the Mach-Zehnder type optical waveguide 310, with a branching ratio of 1:1.

[0184] Further, also reference numeral 340 denotes a 3 dB coupler. The 3 dB coupler 340 functions as a synthesis waveguide on the outgoing side which forms the Mach-Zehnder type optical waveguide 310 and synthesizes lights propagating in the two linear waveguides which form the Mach-Zehnder type optical waveguide 310. The 3 dB coupler 340 outputs the synthesized light as a pair of complementary signals, and one of the complementary signals and the other of the complementary signals are outputted as signal light and monitor light, respectively.

[0185] Similarly, also reference numeral 350 denotes a 3 dB coupler. The 3 dB coupler 350 functions as a branching waveguide on the incoming side which forms the Mach-Zehnder type optical waveguide 320 and branches input light to the linear waveguides, which form the Mach-Zehnder type optical waveguide 320, with a branching ratio of 1:1.

[0186] Further, also reference numeral 360 denotes a 3 dB coupler. The 3 dB coupler 360 functions as a synthesis waveguide on the outgoing side which forms the Mach-Zehnder type optical waveguide 320 and synthesizes light propagating in the two linear waveguides which forms the Mach-Zehnder type optical waveguide 320. The 3 dB coupler outputs the synthesized light as a pair of complementary signals, and one of the complementary signals and the other of the complementary signals are outputted as signal light and monitor light, respectively.

[0187] Further, reference numeral 370 denotes an X dB coupler. The X dB coupler 370 further branches the monitor light branched by the coupler 340 with a desired ratio and emits part of the branched light as light attenuated by X dB to a monitor photodiode 315 side. In other words, the X dB coupler 370 functions as a first coupler provided on the outgoing side of the first Mach-Zehnder type optical waveguide 310 for fetching the monitor light.

[0188] Further, also reference numeral 380 denotes an X dB coupler. The X dB coupler 380 further branches the monitor light branched by the coupler 360 with a desired ratio and emits part of the branched light as light attenuated by X dB to a monitor photodiode 325 side. In other words, the X dB coupler 380 functions as a second coupler provided on the outgoing side of the second Mach-Zehnder type optical waveguide 320 for fetching the monitor light.

[0189] Further, the first monitor photodiode 315 detects a light signal of the monitor light fetched by the 3 dB coupler 340, and the second monitor photodiode 325 detects a light signal of the monitor light fetched by the 3 dB coupler 360.

[0190] Further, a DC (Direct Current) control section 316 serving as a first bias voltage control section controls a bias voltage to be supplied to the first bias electrode section 303 based on the light signal detected by the first monitor photodiode 315. Another DC control section 326 serving as a second bias voltage control section controls a bias voltage to be supplied to the second bias electrode section 305 based on the light signal detected by the second monitor photodiode.

[0191] Configurations of the couplers 330 to 380 are described below in detail.

[0192] All of the couplers 330 to 380 are formed, similarly to those in the optical waveguide device 1 in the first embodiment described above, with the characteristic width according to the present invention. Here, taking notice of the coupler 340, it includes two input waveguides 341 and 342, a waveguide coupling element 345 and two output waveguides 343 and 344.

[0193] Each of the two input waveguides 341 and 342 is supplied with one of different lights from the Mach-Zehnder type optical waveguide 310. In the optical modulator 300 according to the third embodiment, the input waveguide 341 receives light from a linear waveguide 310 b, and the input waveguide 342 receives light from a linear waveguide 310 c.

[0194] The waveguide coupling element 345 couples the two input waveguides 341 and 342 with the distance d₁₁ so that the lights propagating in the two input waveguides interfere with each other. The output waveguides 343 and 344 are branched from the waveguide coupling element 345 with the distance d₂₁. The output waveguide 343 is used as a signal light propagating output waveguide, and the output waveguide 344 is used as a monitoring output waveguide.

[0195] Here, the width on the input side of the waveguide coupling element 345 is greater than the total width of the two input waveguides 341 and 342, and the width on the output side of the waveguide coupling element 345 is greater than the total width of the two output waveguides 343 and 344.

[0196] Further, the distance d₁₁ in the waveguide coupling element 345 is set to one half or less of the width w₀₁₁ at a portion at which the input waveguide 341 (or the input waveguide 342) is connected to the waveguide coupling element 345, and the width w_(w11) of the waveguide coupling element on the input waveguides 341 and 342 side described above is set to 2.5 times to 3.5 times the width w₀₁₁ just described.

[0197] Similarly, the distance d₂₁ in the waveguide coupling element 345 is set to one half or less of the width w₀₂₁ at a portion at which the output waveguide 343 (or the output waveguide 344) is connected to the waveguide coupling element 345, and the width w_(w21) of the waveguide coupling element on the output waveguides 343 and 344 side is set to 2.5 times to 3.5 times the width w₀₂₁ just described.

[0198] Further, taking notice of the coupler 360, it includes two input waveguides 361 and 362, a waveguide coupling element 365 and two output waveguides 363 and 364 which correspond to those (refer to reference numerals 341 to 345) of the coupler 340 described above.

[0199] Here, the waveguide coupling element 365 couples the two input waveguides 361 and 362 with the distance d₁₂ so that lights propagating in the two input waveguides interfere with each other. The output waveguides 363 and 364 are branched from the waveguide coupling element 365 with the distance d₂₂. The output waveguide 363 is used as a signal light propagating output waveguide, and the output waveguide 364 is used as a monitoring output waveguide.

[0200] Here, the width on the input side of the waveguide coupling element 365 is greater than the total width of the two input waveguides 361 and 362, and the width on the output side of the waveguide coupling element 365 is greater than the total width of the output waveguides 363 and 364.

[0201] Further, the distance d₁₂ in the waveguide coupling element 365 is set to one half or less of the width w₀₁₂ at a portion at which the input waveguide 361 (or the input waveguide 362) is connected to the waveguide coupling element 365, and the width w_(w12) of the waveguide coupling element on the input waveguides 361 and 362 side described above is set to 2.5 times to 3.5 times the width w₀₁₂ just described.

[0202] Similarly, the distance d₂₂ in the waveguide coupling element 365 is set to one half or less of the width w₀₂₂ at a portion at which the output waveguide 363 (or the output waveguide 364) is connected to the waveguide coupling element 365, and the width w_(w22) of the waveguide coupling element on the output waveguides 363 and 364 side is set to 2.5 times to 3.5 times the width w₀₂₂ just described.

[0203] Consequently, particularly in each of the 3 dB couplers 340 and 360, generation of radiation mode light when the lights from the linear waveguides which form the Mach-Zehnder type optical waveguides 310 and 320 interfere with each other at the waveguide coupling elements 345 and 365 can be suppressed. Therefore, similarly as in the those of the second embodiment described above, incoincidence between the voltage value V₁ when the signal light is in an on-state and the voltage value V₂ when the monitor light is in an off-state can be suppressed.

[0204] Further, in the 3 dB couplers 330 to 360 described above, the length Lc of the waveguide coupling element (for the coupler 340, refer to reference numeral 345) is set to the length L₂ described hereinabove with reference to FIG. 17. In particular, the length of the waveguide coupling element (345) is set so that light propagating in one of the two input waveguides (similarly refer to reference numerals 341 and 342) may be branched to the two output waveguides (similarly refer to reference numerals 343 and 344) with a power ratio of 1:1.

[0205] Consequently, particularly in the 3 dB couplers 340 and 360 on the outgoing side of the Mach-Zehnder type optical waveguide 310 and 320, one of the output waveguides (for the 3 dB coupler 340, refer to reference numeral 344) can be used as a signal light propagating waveguide, and the other of the output waveguides (for the 3 dB coupler 340, refer to reference numeral 343) can be used as a monitoring waveguide.

[0206] It is to be noted that the coupler 370 further branches the light fetched as monitoring light by the 3 dB coupler 340 and supplies one of the branched lights to the monitor photodiode 315. Further, the length of the waveguide coupling element in the coupler 370 is set so that the branching ratio described above may be a branching ratio with which the branched light can be received with an optimum sensitivity by the monitor photodiode 315 in the next stage.

[0207] Similarly, the coupler 380 further branches the light fetched as monitoring light by the 3 dB coupler 360 and supplies one of the branched lights to the monitor photodiode 325. Further, the length of the waveguide coupling element in the coupler 380 is set so that the branching ratio described above may be a branching ratio with which the branched light can be received with an optimum sensitivity by the monitor photodiode 325 in the next stage.

[0208] Further, also in the couplers 330 to 380 of the optical modulator 300 according to the present embodiment, the distance on the input waveguides side and the distance on the output waveguides side described above are set substantially equal to each other, and the widths of the input and output waveguides, and besides the widths of the waveguide coupling element on the input and output sides, are set substantially equal to each other. In addition, the input waveguides and the output waveguides are connected to the waveguide coupling element with an opening angle of θ=0.5 degrees or more at the connecting portions.

[0209] For example, the coupler 340 is configured such that the distances d₁₁ and d₂₁ described above are set substantially equal to each other, and the widths w₀₁₁ and w₀₂₁, and besides the widths w_(w11) and w_(w21) of the waveguide coupling element, are set substantially equal to each other. In addition, the input waveguide 341 and the output waveguide 342 are connected to the waveguide coupling element 345 with an opening angle of θ=0.5 degrees or more at the connecting portions.

[0210] Further, the coupler 360 is configured such that the distances d₁₂ and d₂₂ described above are set substantially equal to each other, and the widths w₀₁₂ and w₀₂₃, and besides the widths w_(w12) and w_(w22) of the waveguide coupling element, are set substantially equal to each other. In addition, the input waveguides 361 and 362 and the output waveguides 363 and 364 are connected to the waveguide coupling element 365 with an opening angle of θ=0.5 degrees or more at the connecting portions.

[0211] By the configuration described above, in the optical modulator 300 according to the third embodiment of the present invention, first, a voltage of a clock signal from the clock signal supplying source 314 is applied to the first signal electrode section 302 to modulate the light propagating in the first Mach-Zehnder type optical waveguide 310.

[0212] At this time, part of the modulated lights fetched by the couplers 340 and 370 is received as monitor light by the monitor photodiode 315, and a bias voltage corresponding to the reception light signal is applied to the first bias electrode section 303 through the DC control section 316. Consequently, operation of the modulated clock signal light is controlled.

[0213] Then, a voltage of a data signal from the data signal supplying source 324 is applied to the second signal electrode section 304 to modulate the light propagating in the second Mach-Zehnder type optical waveguide 320. Consequently, a signal with which the clock signal and the data signal are superposed and modulated is emitted as signal light.

[0214] At this time, part of the modulated lights fetched by the couplers 360 and 380 is received as monitor light by the monitor photodiode 325, and a bias voltage corresponding to the reception light signal is applied to the second bias electrode section 305 through the DC control section 326. Consequently, operation of the modulated signal light is controlled.

[0215] In other words, application of the bias voltage for controlling the operation of the signal light is performed for each of the modulation processes performed in the Mach-Zehnder type optical waveguides 310 and 320.

[0216] Further, in the 3 dB coupler 340 on the outgoing side of the first Mach-Zehnder type optical waveguide 310, the width on the incoming side (input waveguides 341 and 342 side) is set to the width w_(w11) described above, and the width on the outgoing side (output waveguides 343 and 344 side) is set to the width w_(w21) described above. Consequently, generation of radiation mode light when the lights from the linear waveguides which form the first Mach-Zehnder type optical waveguide 310 interfere with each other at the waveguide coupling element 345 is suppressed so that increase of the insertion loss of light may be suppressed.

[0217] Similarly, in the 3 dB coupler 360 on the outgoing side of the second Mach-Zehnder type optical waveguide 320, the width on the incoming side is set to the width w_(w12) described above, and the width on the outgoing side is set to the width w_(w22) described above. Consequently, generation of radiation mode light when the lights from the linear waveguides which form the second Mach-Zehnder type optical waveguide interfere with each other at the 3 dB coupler 360 is suppressed so that increasing of the insertion loss of light may be suppressed.

[0218] In this manner, with the optical modulator 300 according to the third embodiment of the present invention, there is an advantage that, since the widths on the incoming side of the 3 dB couplers 310 and 320 individually provided on the outgoing sides of the Mach-Zehnder type optical waveguides 310 and 320 are set to the widths w_(w11) and w_(w12) described above and the widths of the waveguide coupling elements on the outgoing side are set to the widths w_(w21) and w_(w22) described above, respectively, generation of radiation mode light when the propagating lights interfere with each other at the waveguide coupling element can be suppressed, and, similarly as in that of the second embodiment described above, incoincidence between the voltage value V₁ when the signal light is in an on-state and the voltage value V₂ when the monitor light is in an off-state, that is, disagreement of the operating points, can be suppressed.

[0219] It is to be noted that, in the optical modulator 300 according to the third embodiment described above, the widths w_(w11) and w_(w12) on the input side and the widths w_(w21) and w_(w22) on the output side of the waveguide coupling element in the couplers 330 to 380 have the characteristic width of the present invention. However, the present invention is not limited to this, but the characteristic width of the present invention can be given at least only to the width of the waveguide coupling element on the input waveguides side or only to the width of the waveguide coupling element on the output waveguides side.

[0220] Further, while, in the present embodiment, the couplers 330 to 380 are configured such that the widths w₀₁₁ to w₀₂₂ and the distances d₁₁ to d₂₂ of the input and output waveguides and the widths w_(w11) to w_(w22) of the waveguide coupling element are substantially equal to each other, according to the present invention, they can be configured otherwise such that the scales of them suitably differ from each other. Also where the characteristic width of the present invention is given only to the width of the waveguide coupling element on the input waveguides side or only to the width of the waveguide coupling element on the output waveguides side, similarly as in the example just described, the scales described above can be set different from each other.

[0221] In this instance, at least in the waveguide coupling elements 345 and 365 of the couplers 340 and 360 which form the multiplexing waveguides of the components of the Mach-Zehnder type optical waveguides 310 and 320, only the widths w_(w11) and w_(w12) can be selectively set to the characteristic scale of the present invention such that the widths w_(w11) and w₁₂ are set to 2.5 times to 3.5 times the widths w₀₁₁ and w₀₁₂, respectively. Otherwise, only the widths w_(w21) and w_(w22) can be set to the characteristic scale of the present invention such that the widths w_(w21) and w_(w22) are set to 2.5 times to 3.5 times the widths w₀₂₁ and w₀₂₂, respectively.

[0222] Further, in each of the couplers 330 to 380, the input waveguides and the output waveguides are connected to the waveguide coupling element with the opening angle θ of 0.5 degrees or more at the connecting portions. However, according to the present invention, even if the opening angle just described is not given, generation of radiation light at least on the incoming side or the outgoing side of the waveguide coupling element can be suppressed.

[0223] Further, in the optical modulator 300 according to the third embodiment described above, the couplers 370 and 380 which function as an attenuator are provided. However, according to the present invention, the optical modulator 300 may be configured otherwise such that the couplers 370 and 380 are omitted but the monitor lights fetched by the couplers 340 and 360 are inputted to the monitor photodiodes 315 and 325, respectively, using some other known attenuation elements.

[0224] [D] Description of the Fourth Embodiment

[0225]FIG. 15 is a block diagram showing an acousto-optical filter apparatus according to a fourth embodiment of the present invention. The acousto-optic tunable filter apparatus 400 shown in FIG. 15 includes first and second acousto-optic tunable filters 410 and 420 formed in series on a substrate 401.

[0226] The first acousto-optic tunable filter 410 includes an optical waveguide 413 and an electrode 402, and the second acousto-optic tunable filter 420 includes an optical waveguide 423 and an electrode 404. The optical waveguide 413 includes a polarization beam splitter 430, a pair of linear waveguides 411 and 412, and another polarization beam splitter 440. The optical waveguide 423 includes a polarization beam splitter 450, a pair of linear waveguides 421 and 422, and another polarization beam splitter 460.

[0227] The polarization beam splitter 430 polarizes and separates propagation light into polarization waves of polarization modes of TM and TE. The two linear waveguides 411 and 412 propagate the propagation lights polarized and separated by the polarization beam splitter 430, and the polarization beam splitter 440 synthesizes the polarization waves of the polarization lights from the linear waveguides 411 and 412.

[0228] Similarly, the polarization beam splitter 450 polarizes and separates the propagation light from the first acousto-optic tunable filter 410 into polarization waves of polarization modes of TM and TE. The two linear waveguides 421 and 422 propagate the propagation lights polarized and separated by the polarization beam splitter 450, and the polarization beam splitter 460 synthesizes the polarization waves of the polarization lights from the linear waveguides 421 and 422.

[0229] Furthermore, RF (Radio Frequency) signals for generating surface acoustic waves for an acousto-optic effect on the optical waveguides 413 and 423 are applied from RF signal sources 403A and 403B to the electrodes 402 and 404 of the first and second acousto-optic tunable filters 410 and 420, respectively.

[0230] Now, configurations of the polarization beam splitters 430 and 450 and the polarization beam splitters 440 and 460 described above are described in more detail.

[0231] The polarization beam splitters 430 and 450 and the polarization beam splitters 440 and 460 described above are formed with the characteristic width according to the present invention similarly to the optical waveguide device 1 in the first embodiment described hereinabove. First, taking notice of the polarization beam splitter 430, it includes two polarization wave separating input waveguides 431 and 432, a polarization wave separating waveguide coupling element 435, and two polarization wave separating output waveguides 433 and 434.

[0232] The polarization wave separating waveguide coupling element 435 couples the two polarization wave separating input waveguides 431 and 432 with a distance d₁₃ so that lights propagating in the two polarization wave separating input waveguides 431 and 432 may interfere with each other. Further, the polarization wave separating output waveguides 433 and 434 branch from the polarization wave separating waveguide coupling element 435 with a distance d₂₃.

[0233] Here, the width on the input side of the waveguide coupling element 435 is greater than the total width of the two input waveguides 431 and 432, and the width on the output side of waveguide coupling element 435 is greater than the total width of the two output waveguides 433 and 434.

[0234] Further, the distance d₁₃ described above is set to one half or less of the width w₀₁₃ at a portion at which each of the polarization wave separating input waveguides 431 and 432 is connected to the polarization wave separating waveguide coupling element 435, and the width w_(w13) of the polarization wave separating waveguide coupling element on the polarization wave separating input waveguides side is set to 2.5 times to 3.5 times the width w₀₁ described hereinabove. Furthermore, the distance d₂₃ described above is set to one half or less of the width w₀₂₃ at a portion at which each of the polarization wave separating output waveguides 433 and 434 is connected to the polarization wave separating waveguide coupling element 435, and the width w_(w23) of the polarization wave separating waveguide coupling element on the polarization wave separating output waveguides side is set to 2.5 times to 3.5 times the width w₀₁ described hereinabove.

[0235] Taking notice of the polarization beam splitter 440, it includes a polarization beam splitter 440, two polarization wave synthesizing input waveguides 441 and 442, a polarization wave synthesizing waveguide coupling element 445, and two polarization wave synthesizing output waveguides 443 and 444.

[0236] The polarization wave synthesizing waveguide coupling element 445 couples the two polarization wave synthesizing input waveguides 441 and 442 with a distance d₁₄ so that lights propagating in the two polarization wave synthesizing input waveguides 441 and 442 may interfere with each other. Further, the polarization wave synthesizing output waveguides 443 and 444 branch from the polarization wave synthesizing waveguide coupling element 445 with a distance d₂₄.

[0237] Further, the distance d₁₄ described above is set to one half or less of the width w₀₁₄ at a portion at which each of the polarization wave synthesizing input waveguides 441 and 442 is connected to the polarization wave synthesizing waveguide coupling element 445, and the width w_(w14) of the polarization wave synthesizing waveguide coupling element on the polarization wave synthesizing input waveguides side is set to 2.5 times to 3.5 times the width w₀₁₄ described above. Furthermore, the distance d₂₄ described above is set to one half or less of the width w₀₂₄ described above at a portion at which each of the polarization wave synthesizing output waveguides 443 and 444 is connected to the polarization wave synthesizing waveguide coupling element 445, and the width w_(w24) of the polarization wave synthesizing waveguide coupling element on the polarization wave synthesizing output waveguides side is set to 2.5 times to 3.5 times the width w₀₂₄ described above.

[0238] Further, in the polarization beam splitters 430 and 450 described above, the length Lc of the waveguide coupling element (for the case of the polarization beam splitter 430, refer to reference character 435) is set so as to be equal to the length L₂ described hereinabove with reference to FIG. 2. In particular, the length of the waveguide coupling element (435) is set so that the light which propagates in one of the two input waveguides (similarly refer to reference characters 343 and 344) is polarized and separated to the two output waveguides (similarly refer to reference characters 343 and 344).

[0239] For example, in the polarization beam splitter 430, if light is inputted from the polarization wave separating input waveguide 431, then it is polarized and separated by the polarization wave separating polarization beam splitter 450 such that TM light can be emitted from the polarization wave separating output waveguide 433 while TE light is outputted from the polarization wave separating output waveguide 434.

[0240] Also each of the polarization beam splitters 430 and 450 of the acousto-optic tunable filter apparatus 400 according to the present embodiment is configured such that the distances d₁₃ and d₂₃ described above are set substantially equal to each other, and the widths w₀₁₃ and w₀₂₃, and besides the widths w_(w13) and w_(w23) of the waveguide coupling element, are set substantially equal to each other. In addition, the input waveguides and the output waveguides are connected to the waveguide coupling element with an opening angle of θ=0.5 degrees or more at the connecting portions.

[0241] Furthermore, also each of the polarization beam splitters 430 and 450 is configured such that the distances d₁₄ and d₂₄ described above are set substantially equal to each other, and the widths w₀₁₄ and w₀₂₄, and besides the widths w_(w14) and w_(w24) of the waveguide coupling element, are set substantially equal to each other. In addition, the input waveguides and the output waveguides are connected to the waveguide coupling element with an opening angle of θ=0.5 degrees or more at the connecting portions.

[0242] By the configuration described above, in the acousto-optic tunable filter apparatus 400 according to the fourth embodiment of the present invention, the first and second acousto-optic tunable filters 410 and 420 vary the refraction factors or the like of polarization waves propagating in the optical waveguides 413 and 423 with RF signals applied to the electrodes 402 and 404 to perform desired filter processing.

[0243] Further, since the polarization beam splitters 430 and 450 are configured such that the width on the incoming side (polarization wave separating input waveguides 431 and 432 side) is set to the width w_(w13) described above and the width on the outgoing side (polarization wave separating output waveguides 433 and 434 side) is set to the width w_(w23) described hereinabove and the polarization beam splitters 440 and 460 are configured such that the width on the incoming side (polarization wave synthesizing input waveguides 441 and 442 side) is set to the width ww₁₄ described hereinabove and the width on the outgoing side (polarization wave separating output waveguides 433 and 434 side) is set to the width w_(w24) described hereinabove, generation of radio mode light in the polarization beam splitters 430 and 450 and the polarization beam splitters 440 and 460 is suppressed.

[0244] In this manner, with the acousto-optic tunable filter apparatus 400 according to the fourth embodiment of the present invention described above, since generation of radiation mode light can be suppressed by the polarization beam splitters 430 to 460, where the polarization beam splitters 430 to 460 are connected at multiple stages particularly as in the acousto-optic tunable filter apparatus 400, the extinction ratio of the polarization beam splitters connected in multiple stages can be improved.

[0245] It is to be noted that, in the optical modulator 400 according to the fourth embodiment described above, both of the widths w_(w13) and w_(w14) on the input side and the widths w_(w23) and w_(w24) on the output side of the waveguide coupling elements in the polarization beam splitters 430 to 460 have the characteristic width of the present invention. However, the present invention is not limited to this, but the characteristic width of the present invention can be given at least only to the width of the waveguide coupling element on the input waveguides side or only to the width of the waveguide coupling element on the output waveguides side.

[0246] Further, while, in the present embodiment, the polarization beam splitters 430 to 460 are each configured such that the widths w₀₁₁ to w₀₂₂ and the distances d₁₁ to d₂₂ of the input and output waveguides and the widths w_(w11) to w_(w22) of the waveguide coupling element are substantially equal to each other, according to the present invention, they can be configured otherwise such that the scales of them suitably differ from each other. Also where the characteristic width of the present invention is given only to the width of the waveguide coupling element on the input waveguides side or only to the width of the waveguide coupling element on the output waveguides side, the scales described above can be set different from each other similarly.

[0247] In this instance, at least in the waveguide coupling elements (for the polarization beam splitter 440, refer to reference numeral 445) of the polarization beam splitters 430 to 460 which form optical waveguides 413 and 423, only the widths w_(w13) and w_(w14) can be selectively set to the characteristic scale of the present invention such that the widths w_(w13) and w_(w14) are set to 2.5 times to 3.5 times the widths w₀₁₃ and w₀₁₄, respectively. Or else, only the widths w_(w23) and w_(w24) can be set to the characteristic scale of the present invention such that the widths w_(w23) and w_(w24) are set to 2.5 times to 3.5 times the widths w₀₂₃ and w₀₂₄, respectively.

[0248] Further, in each of the polarization beam splitters 430 to 460, the input waveguides and the output waveguides are connected to the waveguide coupling element with the opening angle θ of 0.5 degrees or more at the connecting portions. However, according to the present invention, even if the opening angle just described is not given to them, generation of radiation light at least on the incoming side or the outgoing side of the waveguide coupling element can be suppressed.

[0249] [E] Others

[0250] In the embodiments described above, a lithium niobate (LiNbO₃) waveguide is used for the optical waveguides. However, according to the present invention, the optical waveguides are not limited to this, but any other optical waveguide such as a glass substrate or semiconductor substrate may be used for the optical waveguides.

[0251] Particularly, in such an optical waveguide device or an optical modulator as in the first to third embodiments, the substrates 2 and 101 to 301 are not limited to those which have an electro-optic effect. In particular, in order to provide a phase difference between lights which propagate in different optical waveguides, the electro-optic effect need not necessarily be utilized, but, for example, where a SiO₂ substrate is used, such a phase difference between lights as described above can be provided by a thermo-optic effect.

[0252] Further, where the optical waveguide device 1 according to the first embodiment is formed as a polarization beam splitter or is applied as an acousto-optic tunable filter apparatus according to the fourth embodiment, in order to provide a difference in refractive index between TE and TM mode lights efficiently, a material having a high double refraction is selectively used for the substrate.

[0253] Furthermore, while, in the optical modulates according to the second and third embodiments described hereinabove, a bias voltage is applied to each of the bias electrodes 131, 231, 312 and 322, the present invention is not limited to this, but the optical modulations may be configured otherwise such that a bias voltage is superposed on and applied together with a signal voltage to a signal electrode.

[0254] Further, the optical device of the present invention can naturally be applied also to various devices other than such optical modulators and acousto-optic tunable filter apparatus as in the second to fourth embodiments described hereinabove.

[0255] The present invention is not limited to the embodiments specifically described above, and variations and modifications can be made without departing from the scope of the present invention. 

What is claimed is:
 1. An optical waveguide device, comprising: two input waveguides; a waveguide coupling element for coupling said two input waveguides with a first distance; and two output waveguides branched from said waveguide coupling element with a second distance; the first distance in said waveguide coupling element being set to one half or less of width at a portion at which each of said input waveguides is connected to said waveguide coupling element; width of said waveguide coupling element on said input waveguides side being set to 2.5 times to 3.5 times the width at the portion at which each of said input waveguides is connected to said waveguide coupling element.
 2. An optical waveguide device, comprising: two input waveguides; a waveguide coupling element for coupling said two input waveguides with a first distance; and two output waveguides branched from said waveguide coupling element with a second distance; the second distance in said waveguide coupling element being set to one half or less of width at a portion at which each of said output waveguides is connected to said waveguide coupling element; width of said waveguide coupling element on said output waveguides side being set to 2.5 times to 3.5 times the width at the portion at which each of said output waveguides is connected to said waveguide coupling element.
 3. An optical waveguide device, comprising: a substrate having an electro-optical effect; a polarization wave separating waveguide formed on said substrate for separating propagation light into polarization waves of two polarization modes of TM and TE; two linear waveguides formed on said substrate for individually propagating the propagation lights separated by said polarization wave separating waveguide; and a polarization wave synthesis waveguide formed on said substrate for synthesizing the propagation lights from said two linear waveguides; said polarization wave separating waveguide including two input waveguides, a waveguide coupling element, and two output waveguides; width of said waveguide coupling element on the input side being set greater than the width of said two input waveguides; width of said waveguide coupling element on the output side being set greater than the width of said two output waveguides.
 4. An optical modulator, comprising: a substrate; and an optical device provided on said substrate and including a branching waveguide for branching propagation light into two lights, two linear waveguides for propagating the propagation lights branched by said branching waveguide, and a synthesis waveguide for synthesizing the propagation lights from said two linear waveguides such that a phase difference is provided to the propagation lights in said two linear waveguides; said synthesis waveguide including two input waveguides for receiving the propagation lights from said two linear waveguides as inputs thereto, a waveguide coupling element, and two output waveguides; width of said waveguide coupling element on the input side being set greater than the width of said two input waveguides; width of said waveguide coupling element on the output side being set greater than the width of said two output waveguides.
 5. An optical waveguide device, comprising: two input waveguides; a waveguide coupling element for coupling said two input waveguides with a distance d₁; and two output waveguides branched from said waveguide coupling element with a distance d₂; the distance d₁ in said waveguide coupling element being set to one half or less of width w₀₁ of a portion at which each of said input waveguides is connected to said waveguide coupling element; width w_(w1) of said waveguide coupling element on said input waveguides side being set to 2.5 times to 3.5 times the width w₀₁.
 6. The optical waveguide device as claimed in claim 5, wherein the distance d₂ in said waveguide coupling element is set to one half or less of width w₀₂ of a portion at which each of said output waveguides is connected to said waveguide coupling element, and width w_(w2) of said waveguide coupling element on said output waveguides side is set to 2.5 times to 3.5 times the width w₀₂.
 7. An optical waveguide device, comprising: two input waveguides; a waveguide coupling element for coupling said two input waveguides with a distance d₁; and two output waveguides branched from said waveguide coupling element with a distance d₂; the distance d₂ in said waveguide coupling element being set to one half or less of width w₀₂ of a portion at which each of said output waveguides is connected to said waveguide coupling element; width w_(w2) of said waveguide coupling element on said output waveguides side being set to 2.5 times to 3.5 times the width w₀₂.
 8. An optical waveguide device, comprising: two input waveguides; a waveguide coupling element for coupling said two input waveguides with a distance d₁; and two output waveguides branched from said waveguide coupling element with a distance d₂; width w_(w1) of said waveguide coupling element on said input waveguides side being greater than the sum 2w₀₁+d₁ of the sum 2w₀₁ of widths of portions at which said two input waveguides are connected to said waveguide coupling element and the distance d₁ so that radiation mode light can be suppressed better than where the width w_(w1) is equal to the sum 2w₀₁+d₁.
 9. The optical waveguide device as claimed in claim 8, wherein width w_(w2) of said waveguide coupling element on said output waveguides side is greater than the sum 2w₀₂+d₂ of the sum 2w₀₂ of widths of portions at which said two output waveguides are connected to said waveguide coupling element and the distance d₂ so that radiation mode light can be suppressed better than where the width w_(w2) is equal to the sum 2w₀₂+d₂.
 10. An optical waveguide device, comprising: two input waveguides; a waveguide coupling element for coupling said two input waveguides with a distance d₁; and two output waveguides branched from said waveguide coupling element with a distance d₂; width w_(w2) of said waveguide coupling element on said output waveguides side being greater than the sum 2w₀₂+d₂ of the sum 2w₀₂ of widths of portions at which said two output waveguides are connected to said waveguide coupling element and the distance d₂ so that radiation mode light can be suppressed better than where the width w_(w2) is equal to the sum 2w₀₂+d₂.
 11. The optical waveguide device as claimed in claim 5, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 12. The optical waveguide device as claimed in claim 6, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 13. The optical waveguide device as claimed in claim 7, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 14. The optical waveguide device as claimed in claim 8, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 15. The optical waveguide device as claimed in claim 10, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 16. The optical waveguide device as claimed in claim 5, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 17. The optical waveguide device as claimed in claim 6, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 18. The optical waveguide device as claimed in claim 7, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 19. The optical waveguide device as claimed in claim 8, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 20. The optical waveguide device as claimed in claim 10, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 21. The optical waveguide device as claimed in claim 5, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 22. The optical waveguide device as claimed in claim 6, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 23. The optical waveguide device as claimed in claim 7, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 24. The optical waveguide device as claimed in claim 8, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 25. The optical waveguide device as claimed in claim 10, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 26. The optical waveguide device as claimed in claim 5, wherein the length of said waveguide coupling element is set so that, where a polarization wave of one of two polarization modes of TM and TE is inputted from one of said two input waveguides, the polarization wave may be branched with a desired branching ratio to said two output waveguides.
 27. The optical waveguide device as claimed in claim 6, wherein the length of said waveguide coupling element is set so that, where a polarization wave of one of two polarization modes of TM and TE is inputted from one of said two input waveguides, the polarization wave may be branched with a desired branching ratio to said two output waveguides.
 28. The optical waveguide device as claimed in claim 7, wherein the length of said waveguide coupling element is set so that, where a polarization wave of one of two polarization modes of TM and TE is inputted from one of said two input waveguides, the polarization wave may be branched with a desired branching ratio to said two output waveguides.
 29. The optical waveguide device as claimed in claim 8, wherein the length of said waveguide coupling element is set so that, where a polarization wave of one of two polarization modes of TM and TE is inputted from one of said two input waveguides, the polarization wave may be branched with a desired branching ratio to said two output waveguides.
 30. The optical waveguide device as claimed in claim 10, wherein the length of said waveguide coupling element is set so that, where a polarization wave of one of two polarization modes of TM and TE is inputted from one of said two input waveguides, the polarization wave may be branched with a desired branching ratio to said two output waveguides.
 31. The optical waveguide device as claimed in claim 5, wherein the length of said waveguide coupling element is set so that, where polarization waves of two polarization modes of TM and TE are inputted from one of said two input waveguides, each of the polarization waves may be branched with a desired branching ratio to said two output waveguides.
 32. The optical waveguide device as claimed in claim 6, wherein a length of said waveguide coupling element is set so that, where polarization waves of two polarization modes of TM and TE are inputted from one of said two input waveguides, each of the polarization waves maybe branched with a desired branching ratio to said two output waveguides.
 33. The optical waveguide device as claimed in claim 7, wherein a length of said waveguide coupling element is set so that, where polarization waves of two polarization modes of TM and TE are inputted from one of said two input waveguides, each of the polarization waves maybe branched with a desired branching ratio to said two output waveguides.
 34. The optical waveguide device as claimed in claim 8, wherein a length of said waveguide coupling element is set so that, where polarization waves of two polarization modes of TM and TE are inputted from one of said two input waveguides, each of the polarization waves may be branched with a desired branching ratio to said two output waveguides.
 35. The optical waveguide device as claimed in claim 10, wherein a length of said waveguide coupling element is set so that, where polarization waves of two polarization modes of TM and TE are inputted from one of said two input waveguides, each of the polarization waves may be branched with a desired branching ratio to said two output waveguides.
 36. An optical waveguide device, comprising: a substrate having an electro-optical effect; and a Mach-Zehnder type optical waveguide formed on said substrate and including a branching waveguide for branching propagation light into two propagation lights, two linear waveguides for propagating the two propagation lights branched by said branching waveguide, and a multiplexing waveguide for multiplexing the propagation lights from said two linear waveguides; said multiplexing waveguide including two input waveguides for inputting the propagation lights from said linear waveguides, a waveguide coupling element for coupling said two input waveguides with a distance d₁, and two output waveguides branched from said waveguide coupling element with another distance d₂; the distance d₁ in said waveguide coupling element being set to one half or less of width w₀₁ at a portion at which each of said input waveguides is connected to said waveguide coupling element; width w_(w1) of said waveguide coupling element on said input waveguides side being set to 2.5 times to 3.5 times the width w₀₁; the length of said waveguide coupling element being set so that light propagating in one of said two input waveguides is branched to said two output waveguides with a power ratio of 1:1.
 37. An optical waveguide device, comprising: a substrate having an electro-optical effect; and a Mach-Zehnder type optical waveguide formed on said substrate and including a branching waveguide for branching propagation light into two propagation lights, two linear waveguides for propagating the two propagation lights branched by said branching waveguide, and a multiplexing waveguide for multiplexing the propagation lights from said two linear waveguides; said multiplexing waveguide including two input waveguides for inputting the propagation lights from said linear waveguides, a waveguide coupling element for coupling said two input waveguides with distance d₁, and two output waveguides branched from said waveguide coupling element with another distance d₂; the distance d₂ in said waveguide coupling element being set to one half or less of width w₀₂ at a portion at which each of said output waveguides is connected to said waveguide coupling element; width w_(w2) of said waveguide coupling element on said output waveguides side being set to 2.5 times to 3.5 times the width w₀₂; the length of said waveguide coupling element being set so that light propagating in one of said two input waveguides is branched to said two output waveguides with a power ratio of 1:1.
 38. The optical waveguide device as claimed in claim 36, wherein one of said two output waveguides is used as a signal light propagating waveguide, and the other of said two output waveguides is used as a monitoring waveguide.
 39. The optical waveguide device as claimed in claim 37, wherein one of said two output waveguides is used as a signal light propagating waveguide, and the other of said two output waveguides is used as a monitoring waveguide.
 40. An optical waveguide device, comprising: a substrate having an electro-optical effect; a polarization wave separating waveguide formed on said substrate for separating propagation light into polarization waves of polarization modes of TM and TE; two linear waveguides formed on said substrate for individually propagating the propagation lights separated by said polarization wave separating waveguide; and a polarization wave synthesis waveguide formed on said substrate for synthesizing the propagation lights from said two linear waveguides; said polarization wave separating waveguide including two input waveguides, a waveguide coupling element for coupling said two input waveguides with a distance d₁, and two output waveguides branched from said waveguide coupling element with another distance d₂; the distance d₁ in said waveguide coupling element being set to one half or less of width w₀₁ at a portion at which each of said input waveguides is connected to said waveguide coupling element; width w_(w1) of said waveguide coupling element on said input waveguides side being set to 2.5 times to 3.5 times the width w₀₁; the length of said waveguide coupling element being set so that, where polarization waves of the two polarization modes of TM and TE are inputted from one of said two input waveguides, the polarization waves may individually be outputted to different ones of said output waveguides from each other.
 41. An optical waveguide device, comprising: a substrate having an electro-optical effect; a polarization wave separating waveguide formed on said substrate for separating propagation light into polarization waves of polarization modes of TM and TE; two linear waveguides formed on said substrate for individually propagating the propagation lights separated by said polarization wave separating waveguide; and a polarization wave synthesis waveguide formed on said substrate for synthesizing the propagation lights from said two linear waveguides; said polarization wave separating waveguide including two input waveguides, a waveguide coupling element for coupling said two input waveguides with a distance d₁, and two output waveguides branched from said waveguide coupling element with another distance d₂; the distance d₂ in said waveguide coupling element being set to one half or less of width w₀₂ at a portion at which each of said output waveguides is connected to said waveguide coupling element; width w_(w2) of said waveguide coupling element on said output waveguides side being set to 2.5 times to 3.5 times the width w₀₂; the length of said waveguide coupling element being set so that, where polarization waves of the two polarization modes of TM and TE are inputted from one of said two input waveguides, the polarization waves may individually be outputted to different ones of said output waveguides from each other.
 42. An optical waveguide device, comprising: a substrate having an electro-optical effect; a polarization wave separating waveguide formed on said substrate for separating propagation light into polarization waves of polarization modes of TM and TE; two linear waveguides formed on said substrate for individually propagating the propagation lights separated by said polarization wave separating waveguide; and a polarization wave synthesis waveguide formed on said substrate for synthesizing the propagation lights from said two linear waveguides; said polarization wave synthesis waveguide including two input waveguides for receiving the propagation lights from said two linear waveguides as inputs thereto, a waveguide coupling element for coupling said two input waveguides with a distance d₁, and two output waveguides branched from said waveguide coupling element with another distance d₂; the distance d₁ in said waveguide coupling element being set to one half or less of width w₀₁ at a portion at which each of said input waveguides is connected to said waveguide coupling element; width w_(w1) of said waveguide coupling element on said input waveguides side being set to 2.5 times to 3.5 times the width w₀₁; the length of said waveguide coupling element being set so that, where a polarization wave of the polarization mode of TM is inputted from one of said two input waveguides and another polarization wave of the polarization mode of TE is inputted from the other of said two input waveguides, the polarization waves maybe synthesized by said waveguide coupling element and the synthesized polarization light may be outputted to one of said output waveguides.
 43. An optical waveguide device, comprising: a substrate having an electro-optical effect; a polarization wave separating waveguide formed on said substrate for separating propagation light into polarization waves of polarization modes of TM and TE; two linear waveguides formed on said substrate for individually propagating the propagation lights separated by said polarization wave separating waveguide; and a polarization wave synthesis waveguide formed on said substrate for synthesizing the propagation lights from said two linear waveguides; said polarization wave synthesis waveguide including two input waveguides for receiving the propagation lights from said two linear waveguides as inputs thereto, a waveguide coupling element for coupling said two input waveguides with a distance d₁, and two output waveguides branched from said waveguide coupling element with another distance d₂; the distance d₂ in said waveguide coupling element being set to one half or less of width w₀₂ at a portion at which each of said output waveguides is connected to said waveguide coupling element; width w_(w2) of said waveguide coupling element on said output waveguides side being set to 2.5 times to 3.5 times the width w₀₂; the length of said waveguide coupling element being set so that, where a polarization wave of the polarization mode of TM is inputted from one of said two input waveguides and another polarization wave of the polarization mode of TE is inputted from the other of said two input waveguides, the polarization waves may be synthesized by said waveguide coupling element and the synthesized polarization wave may be outputted from one of said output waveguides.
 44. The optical waveguide device as claimed in claim 36, wherein the distance d₂ in said waveguide coupling element is set to one half or less of the width w₀₂ at a portion at which each of said output waveguides is connected to said waveguide coupling element, and the width w_(w2) of said waveguide coupling element on said output waveguides side is set to 2.5 times to 3.5 times the width w₀₂.
 45. The optical waveguide device as claimed in claim 40, wherein the distance d₂ in said waveguide coupling element is set to one half or less of the width w₀₂ at a portion at which each of said output waveguides is connected to said waveguide coupling element, and the width w_(w2) of said waveguide coupling element on said output waveguides side is set to 2.5 times to 3.5 times the width w₀₂.
 46. The optical waveguide device as claimed in claim 42, wherein the distance d₂ in said waveguide coupling element is set to one half or less of the width w₀₂ at a portion at which each of said output waveguides is connected to said waveguide coupling element, and the width w_(w2) of said waveguide coupling element on said output waveguides side is set to 2.5 times to 3.5 times the width w₀₂.
 47. The optical waveguide device as claimed in claim 36, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 48. The optical waveguide device as claimed in claim 37, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 49. The optical waveguide device as claimed in claim 40, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 50. The optical waveguide device as claimed in claim 41, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 51. The optical waveguide device as claimed in claim 42, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 52. The optical waveguide device as claimed in claim 43, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 53. The optical waveguide device as claimed in claim 36, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 54. The optical waveguide device as claimed in claim 37, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 55. The optical waveguide device as claimed in claim 40, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 56. The optical waveguide device as claimed in claim 41, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 57. The optical waveguide device as claimed in claim 42, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 58. The optical waveguide device as claimed in claim 43, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 59. The optical waveguide device as claimed in claim 36, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 60. The optical waveguide device as claimed in claim 37, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 61. The optical waveguide device as claimed in claim 40, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 62. The optical waveguide device as claimed in claim 41, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 63. The optical waveguide device as claimed in claim 42, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 64. The optical waveguide device as claimed in claim 43, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 65. An optical modulator, comprising: a substrate having an electro-optical effect; a Mach-Zehnder type optical waveguide formed on said substrate; a signal electrode section provided on said optical waveguide for being supplied with a signal voltage; a bias electrode section provided on said optical waveguide for being supplied with a bias voltage; a coupler provided on the outgoing side of said Mach-Zehnder type optical waveguide for fetching monitor light; a monitor photodiode for detecting a light signal of the monitor light fetched by said coupler; and a bias voltage control section for controlling the bias voltage to be supplied to said bias electrode section based on the light signal detected by said monitor photodiode; said coupler including two input waveguides through one of which light from said Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling said two input waveguides with a distance d₁, and a signal light propagating output waveguide and a monitoring output waveguide branched from said waveguide coupling element with a distance d₂; the distance d₁ in said waveguide coupling element being set to one half or less of width w₀₁ at a portion at which each of said input waveguides is connected to said waveguide coupling element; width w_(w1) of said waveguide coupling element on said input waveguides side being set to 2.5 times to 3.5 times the width w₀₁; the length of said waveguide coupling element being set so that light which propagates in one of said two input waveguides may be branched to said two output waveguides with a power ratio of 1:1.
 66. An optical modulator, comprising: a substrate having an electro-optical effect; a Mach-Zehnder type optical waveguide formed on said substrate; a signal electrode section provided on said optical waveguide for being supplied with a signal voltage; a bias electrode section provided on said optical waveguide for being supplied with a bias voltage; a coupler provided on the outgoing side of said Mach-Zehnder type optical waveguide for fetching monitor light; a monitor photodiode for detecting a light signal of the monitor light fetched by said coupler; and a bias voltage control section for controlling the bias voltage to be supplied to said bias electrode section based on the light signal detected by said monitor photodiode; said coupler including two input waveguides through one of which light from said Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling said two input waveguides with a distance d₁, and a signal light propagating output waveguide and a monitoring output waveguide branched from said waveguide coupling element with a distance d₂; the distance d₂ in said waveguide coupling element being set to one half or less of width w₀₂ at a portion at which each of said output waveguides are connected to said waveguide coupling element; width w_(w2) of said waveguide coupling element on said input waveguides side being set to 2.5 times to 3.5 times the width w₀₂; the length of said waveguide coupling element being set so that light which propagates one of said two input waveguides may be branched to said two output waveguides with a power ratio of 1:1.
 67. The optical modulator as claimed in claim 65, wherein said Mach-Zehnder type optical waveguide includes a branching waveguide for branching the propagation light into two lights, two linear waveguides for propagating the two propagation lights branched by said branching waveguide, and a multiplexing waveguide for multiplexing the propagation lights from said two linear waveguides; said signal electrode section including a signal electrode and a ground electrode formed on said two linear waveguides; said bias electrode section including: a first potential supplying electrode section for being supplied with a first potential; a second potential supplying electrode section for being supplied with a second potential which is a potential different from the first potential such that a potential difference between the first and second potentials is used as the bias voltage; a first waveguide electrode section for supplying the first potential from said first potential supplying electrode section to one of said two linear waveguides; a second waveguide electrode section for supplying the second potential from said second potential supplying electrode section to the other of said two linear waveguides; and at least one potential difference producing electrode section for supplying the potential from one of said first and second potential supplying electrode sections to the proximity of said waveguide electrode section for supplying the potential from the other of said first and second potential supplying electrode sections.
 68. The optical modulator as claimed in claim 66, wherein said Mach-Zehnder type optical waveguide includes a branching waveguide for branching the propagation light into two lights, two linear waveguides for propagating the two propagation lights branched by said branching waveguide, and a multiplexing waveguide for multiplexing the propagation lights from said two linear waveguides; said signal electrode section including a signal electrode formed on each of said two linear waveguides and a ground electrode; said bias electrode including: a first potential supplying electrode section for being supplied with a first potential; a second potential supplying electrode section for being supplied with a second potential which is a potential different from the first potential such that a potential difference between the first and second potentials is used as the bias voltage; a first waveguide electrode section for supplying the first potential from said first potential supplying electrode section to one of said two linear waveguides; a second waveguide electrode section for supplying the second potential from said second potential supplying electrode section to the other of said two linear waveguides; at least one potential difference producing electrode section for supplying the potential from one of said first and second potential supplying electrode sections in the proximity of said waveguide electrode section for supplying the potential from the other of said first and second potential supplying electrode sections.
 69. The optical modulator as claimed in claim 65, wherein the distance d₂ in said waveguide coupling element is set to one half or less of the width w₀₂ at the portion at which each of said output waveguides is connected to said waveguide coupling element, and the width w_(w2) of said waveguide coupling element on said output waveguides side is set to 2.5 times to 3.5 times the width w₀₂.
 70. The optical modulator as claimed in claim 65, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 71. The optical modulator as claimed in claim 66, wherein the distances d₁ and d₂ are substantially equal to each other, and the widths w₀₁ and w₀₂ are substantially equal to each other.
 72. The optical modulator as claimed in claim 65, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 73. The optical modulator as claimed in claim 66, wherein the widths w_(w1) and w_(w2) of said waveguide coupling element are substantially equal to each other.
 74. The optical modulator as claimed in claim 65, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 75. The optical modulator as claimed in claim 66, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 76. An optical modulator, comprising: a substrate having an electro-optical effect; first and second Mach-Zehnder type optical waveguides formed in series on said substrate; a first signal electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a clock signal; a second signal electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with a data signal; a first bias electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a bias voltage; a second bias electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with another bias voltage; a first coupler provided on the outgoing side of said first Mach-Zehnder type optical waveguide for fetching monitor light; a second coupler provided on the outgoing side of said second Mach-Zehnder type optical waveguide for fetching monitor light; a first monitor photodiode for detecting a light signal of the monitor light fetched by said first coupler; a second monitor photodiode for detecting a light signal of the monitor light fetched by said second coupler; a first bias voltage control section for controlling the bias voltage to be supplied to said first bias electrode section based on the light signal detected by said first monitor photodiode; and a second bias voltage control section for controlling the bias voltage to be supplied to said second bias electrode section based on the light signal detected by said second monitor photodiode; said first coupler including two input waveguides through one of which light from said first Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling said two input waveguides with a distance d₁₁, and two output waveguides branched from said waveguide coupling element with a distance d₂₁; the distance d₁₁ in said waveguide coupling element of said first coupler being set to one half or less of width w₀₁₁ at a portion at which each of said input waveguides is connected to said waveguide coupling element; width w_(w11) of said waveguide coupling element on said input waveguides side being set to 2.5 times to 3.5 times the width w₀₁₁.
 77. An optical modulator, comprising: a substrate having an electro-optical effect; first and second Mach-Zehnder type optical waveguides formed in series on said substrate; a first signal electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a clock signal; a second signal electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with a data signal; a first bias electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a bias voltage; a second bias electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with another bias voltage; a first coupler provided on the outgoing side of said first Mach-Zehnder type optical waveguide for fetching monitor light; a second coupler provided on the outgoing side of said second Mach-Zehnder type optical waveguide for fetching monitor light; a first monitor photodiode for detecting a light signal of the monitor light fetched by said first coupler; a second monitor photodiode for detecting a light signal of the monitor light fetched by said second coupler; a first bias voltage control section for controlling the bias voltage to be supplied to said first bias electrode section based on the light signal detected by said first monitor photodiode; and a second bias voltage control section for controlling the bias voltage to be supplied to said second bias electrode section based on the light signal detected by said second monitor photodiode; said first coupler including two input waveguides through one of which light from said first Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling said two input waveguides with a distance d₁₁, and two output waveguides branched from said waveguide coupling element with a distance d₂₁; the distance d₂₁ in said waveguide coupling element of said first coupler being set to one half or less of width w₀₂₁ at a portion at which each of said output waveguides is connected to said waveguide coupling element; width w_(w21) of said waveguide coupling element on said output waveguides side being set to 2.5 times to 3.5 times the width w₀₂₁.
 78. An optical modulator, comprising: a substrate having an electro-optical effect; first and second Mach-Zehnder type optical waveguides formed in series on said substrate; a first signal electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a clock signal; a second signal electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with a data signal; a first bias electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a bias voltage; a second bias electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with another bias voltage; a first coupler provided on the outgoing side of said first Mach-Zehnder type optical waveguide for fetching monitor light; a second coupler provided on the outgoing side of said second Mach-Zehnder type optical waveguide for fetching monitor light; a first monitor photodiode for detecting a light signal of the monitor light fetched by said first coupler; a second monitor photodiode for detecting a light signal of the monitor light fetched by said second coupler; a first bias voltage control section for controlling the bias voltage to be supplied to said first bias electrode section based on the light signal detected by said first monitor photodiode; and a second bias voltage control section for controlling the bias voltage to be supplied to said second bias electrode section based on the light signal detected by said second monitor photodiode; said second coupler including two input waveguides through one of which light from said second Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling said two input waveguides with a distance d₁₂, and two output waveguides branched from said waveguide coupling element with a distance d₂₂; the distance d₁₂ in said waveguide coupling element of said second coupler being set to one half or less of width w₀₁₂ at a portion at which said input waveguides are connected to said waveguide coupling element; width w_(w12) of said waveguide coupling element on said input waveguides side being set to 2.5 times to 3.5 times the width w₀₁₂.
 79. An optical modulator, comprising: a substrate having an electro-optical effect; first and second Mach-Zehnder type optical waveguides formed in series on said substrate; a first signal electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a clock signal; a second signal electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with a data signal; a first bias electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a bias voltage; a second bias electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with another bias voltage; a first coupler provided on the outgoing side of said first Mach-Zehnder type optical waveguide for fetching monitor light; a second coupler provided on the outgoing side of said second Mach-Zehnder type optical waveguide for fetching monitor light; a first monitor photodiode for detecting a light signal of the monitor light fetched by said first coupler; a second monitor photodiode for detecting a light signal of the monitor light fetched by said second coupler; a first bias voltage control section for controlling the bias voltage to be supplied to said first bias electrode section based on the light signal detected by said first monitor photodiode; and a second bias voltage control section for controlling the bias voltage to be supplied to said second bias electrode section based on the light signal detected by said second monitor photodiode; said second coupler including two input waveguides through one of which light from said second Mach-Zehnder type optical waveguide is inputted, a waveguide coupling element for coupling said two input waveguides with a distance d₁₂, and two output waveguides branched from said waveguide coupling element with a distance d₂₂; the distance d₂₂ in said waveguide coupling element of said second coupler being set to one half or less of width w₀₂₂ at a portion at which each of said output waveguides is connected to said waveguide coupling element; width w_(w22) of said waveguide coupling element on said output waveguides side being set to 2.5 times to 3.5 times the width w₀₂₂.
 80. An optical modulator, comprising: a substrate having an electro-optical effect; first and second Mach-Zehnder type optical waveguides formed in series on said substrate; a first signal electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a clock signal; a second signal electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with a data signal; a first bias electrode section provided on said first Mach-Zehnder type optical waveguide for being supplied with a bias voltage; a second bias electrode section provided on said second Mach-Zehnder type optical waveguide for being supplied with another bias voltage; a first coupler provided on the outgoing side of said first Mach-Zehnder type optical waveguide for fetching monitor light; a second coupler provided on the outgoing side of said second Mach-Zehnder type optical waveguide for fetching monitor light; a first monitor photodiode for detecting a light signal of the monitor light fetched by said first coupler; a second monitor photodiode for detecting a light signal of the monitor light fetched by said second coupler; a first bias voltage control section for controlling the bias voltage to be supplied to said first bias electrode section based on the light signal detected by said first monitor photodiode; and a second bias voltage control section for controlling the bias voltage to be supplied to said second bias electrode section based on the light signal detected by said second monitor photodiode; said first coupler including two first input waveguides through one of which light from said first Mach-Zehnder type optical waveguide is inputted, a first waveguide coupling element for coupling said two first input waveguides with a distance d₁₁, and two first output waveguides branched from said first waveguide coupling element with a distance d₂₁; said second coupler including two second input waveguides through one of which light from said second Mach-Zehnder type optical waveguide is inputted, a second waveguide coupling element for coupling said two second input waveguides with a distance d₁₂, and two second output waveguides branched from said second waveguide coupling element with a distance d₂₂; the distances d₁₁ and d₂₁ in said first waveguide coupling element of said first coupler being set to one half or less of widths w₀₁₁ and w₀₂₁ at portions at which each of said first input waveguides and each of said first output waveguides are connected to said first waveguide coupling element, respectively; the widths w_(w11) and w_(w21) of said first waveguide coupling element on said first input waveguides side and said first output waveguides side being set to 2.5 times to 3.5 times the widths w₀₁₁ and w₀₂₁, respectively; the distances d₁₂ and d₂₂ in said second waveguide coupling element of said second coupler being set to one half or less of widths w₀₁₂ and w₀₂₂ at portions at which each of said second input waveguides and each of said second output waveguides are connected to said second waveguide coupling element, respectively; the widths w_(w12) and w_(w22) of said second waveguide coupling element on said second input waveguides side and said second output waveguides side being set to 2.5 times to 3.5 times the widths w₀₁₂ and w₀₂₂, respectively.
 81. The optical modulator as claimed in claim 76, wherein the length of said first waveguide coupling element is set so that said two first output waveguides in said first coupler may branch the light from said first waveguide coupling element with a power ratio of 1:1 without depending upon the polarization, and one of said two first output waveguides is used as a signal light propagating waveguide while the other of said two first output waveguides is used as a monitoring waveguide.
 82. The optical modulator as claimed in claim 77, wherein the length of said first waveguide coupling element is set so that said two first output waveguides in said first coupler may branch the light from said first waveguide coupling element with a power ratio of 1:1 without depending upon the polarization, and one of said two first output waveguides is used as a signal light propagating waveguide while the other of said two first output waveguides is used as a monitoring waveguide.
 83. The optical modulator as claimed in claim 80, wherein the length of said first waveguide coupling element is set so that said two first output waveguides in said first coupler may branch the light from said first waveguide coupling element with a power ratio of 1:1 without depending upon the polarization, and one of said two first output waveguides is used as a signal light propagating waveguide while the other of said two first output waveguides is used as a monitoring waveguide.
 84. The optical modulator as claimed in claim 78, wherein the length of said second waveguide coupling element is set so that said two second output waveguides in said second coupler may branch the light from said second waveguide coupling element with a power ratio of 1:1 without depending upon the polarization, and one of said two second output waveguides is used as a signal light propagating waveguide while the other of said two second output waveguides is used as a monitoring waveguide.
 85. The optical modulator as claimed in claim 79, wherein the length of said second waveguide coupling element is set so that said two second output waveguides in said second coupler may branch the light from said second waveguide coupling element with a power ratio of 1:1 without depending upon the polarization, and one of said two second output waveguides is used as a signal light propagating waveguide while the other of said two second output waveguides is used as a monitoring waveguide.
 86. The optical modulator as claimed in claim 80, wherein the length of said second waveguide coupling element is set so that said two second output waveguides in said second coupler may branch the light from said second waveguide coupling element with a power ratio of 1:1 without depending upon the polarization, and one of said two second output waveguides is used as a signal light propagating waveguide while the other of said two second output waveguides is used as a monitoring waveguide.
 87. The optical modulator as claimed in claim 76, wherein the distances d₁₁ and d₂₁ or the distances d₁₂ and d₂₂ are substantially equal to each other, and the widths w₀₁₁ and w₀₂₁ or the widths w₀₁₂ and w₀₂₂ are substantially equal to each other.
 88. The optical modulator as claimed in claim 77, wherein the distances d₁₁ and d₂₁ or the distances d₁₂ and d₂₂ are substantially equal to each other, and the widths w₀₁₁ and w₀₂₁ or the widths w₀₁₂ and w₀₂₂ are substantially equal to each other.
 89. The optical modulator as claimed in claim 78, wherein the distances d₁₁ and d₂₁ or the distances d₁₂ and d₂₂ are substantially equal to each other, and the widths w₀₁₁ and w₀₂₁ or the widths w₀₁₂ and w₀₂₂ are substantially equal to each other.
 90. The optical modulator as claimed in claim 79, wherein the distances d₁₁ and d₂₁ or the distances d₁₂ and d₂₂ are substantially equal to each other, and the widths w₀₁₁ and w₀₂₁ or the widths w₀₁₂ and w₀₂₂ are substantially equal to each other.
 91. The optical modulator as claimed in claim 80, wherein the distances d₁₁ and d₂₁ or the distances d₁₂ and d₂₂ are substantially equal to each other, and the widths w₀₁₁ and w₀₂₁ or the widths w₀₁₂ and w₀₂₂ are substantially equal to each other.
 92. The optical modulator as claimed in claim 76, wherein the widths w_(w11) and w_(w21) or the widths w_(w12) and w_(w22) of said waveguide coupling element are substantially equal to each other.
 93. The optical modulator as claimed in claim 77, wherein the widths w_(w11) and w_(w21) or the widths w_(w12) and w_(w22) of said waveguide coupling element are substantially equal to each other.
 94. The optical modulator as claimed in claim 78, wherein the widths w_(w11) and w_(w21) or the widths w_(w12) and w_(w22) of said waveguide coupling element are substantially equal to each other.
 95. The optical modulator as claimed in claim 79, wherein the widths w_(w11) and w_(w21) or the widths w_(w12) and w_(w22) of said waveguide coupling element are substantially equal to each other.
 96. The optical modulator as claimed in claim 80, wherein the widths w_(w11) and w_(w21) or the widths w_(w12) and w_(w22) of said waveguide coupling element are substantially equal to each other.
 97. The optical modulator as claimed in claim 76, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 98. The optical modulator as claimed in claim 77, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 99. The optical modulator as claimed in claim 78, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 100. The optical modulator as claimed in claim 79, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 101. The optical modulator as claimed in claim 80, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions.
 102. An acousto-optic tunable filter apparatus, comprising: a substrate; and first and second acousto-optic tunable filters formed in series on said substrate and utilizing an acousto-optic effect; each of said first and second acousto-optic tunable filters including an optical waveguide which in turn, includes a first polarization beam splitter for separating propagation light into polarization waves of polarization modes of TM and TE, two linear waveguides for individually propagating the propagation lights separated by said first polarization beam splitter, and a second polarization beam splitter for synthesizing the propagation lights from said two linear waveguides; each of said first and second acousto-optic tunable filters further including an electrode for receiving an RF signal to be used to generate a surface acoustic wave for the acousto-optic effect on said optical waveguide; said first polarization beam splitter including two polarization wave separating input waveguides, a polarization wave separating waveguide coupling element for coupling said two polarization wave separating input waveguides with a distance d₁₃ so that lights propagating in said two polarization wave separating input waveguides interfere with each other, and two polarization wave separating output waveguides branched from said polarization wave separating waveguide coupling element with a distance d₂₃; said second polarization beam splitter including two polarization wave synthesizing input waveguides, a polarization wave synthesizing waveguide coupling element for coupling said two polarization wave synthesizing input waveguides with a distance d₁₄ so that lights propagating in said two polarization wave synthesizing input waveguides interfere with each other, and two polarization wave synthesizing output waveguides branched from said polarization wave synthesizing waveguide coupling element with a distance d₂₄; the distances d₁₃ and d₂₃ being set to one half or less of widths w₀₁₃ and w₀₂₃ at portions at which each of said polarization wave separating input waveguides and each of said polarization wave separating output waveguides are connected to said polarization wave separating waveguide coupling element, respectively; widths w_(w13) and w_(w23) of said polarization wave separating waveguide coupling element on said polarization wave separating input waveguides and said polarization wave separating output waveguides sides being set to 2.5 times to 3.5 times the widths w₀₁₃ and w₀₂₃, respectively; the distances d₁₄ and d₂₄ being set to one half or less of widths w₀₁₄ and w₀₂₄ at portions at which each of said polarization wave synthesizing input waveguides and each of said polarization wave synthesizing output waveguides are connected to said polarization wave synthesizing waveguide coupling element, respectively; widths w_(w14) and w_(w24) of said polarization wave synthesizing waveguide coupling element on said polarization wave synthesizing input waveguides and said polarization wave synthesizing output waveguides sides being set to 2.5 times to 3.5 times the widths w₀₁₄ and w₀₂₄, respectively.
 103. The acousto-optic tunable filter apparatus as claimed in claim 102, wherein the distances d₁₃ and d₂₃ or the distances d₁₄ and d₂₄ are substantially equal to each other, and the widths w₀₁₃ and w₀₂₃ or the widths w₀₁₄ and w₀₂₄ are substantially equal to each other.
 104. The acousto-optic tunable filter apparatus as claimed in claim 102, wherein the widths w_(w13) and w_(w23) of said waveguide coupling element or the widths w_(w14) and w_(w24) of said waveguide coupling element are substantially equal to each other.
 105. The acousto-optic tunable filter apparatus as claimed in claim 102, wherein said input waveguides and output waveguides are connected to said waveguide coupling element with an opening angle of 0.5 degrees or more at the connecting portions. 